diff --git a/staging/src/k8s.io/pod-security-admission/policy/checks.go b/staging/src/k8s.io/pod-security-admission/policy/checks.go
new file mode 100644
index 00000000000..f46b3b3e4ac
--- /dev/null
+++ b/staging/src/k8s.io/pod-security-admission/policy/checks.go
@@ -0,0 +1,178 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package policy
+
+import (
+	"strings"
+
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/pod-security-admission/api"
+)
+
+type Check struct {
+	// ID is the unique ID of the check.
+	ID string
+	// Level is the policy level this check belongs to.
+	// Must be Baseline or Restricted.
+	// Baseline checks are evaluated for baseline and restricted namespaces.
+	// Restricted checks are only evaluated for restricted namespaces.
+	Level api.Level
+	// Versions contains one or more revisions of the check that apply to different versions.
+	// If the check is not yet assigned to a version, this must be a single-item list with a MinimumVersion of "".
+	// Otherwise, MinimumVersion of items must represent strictly increasing versions.
+	Versions []VersionedCheck
+}
+
+type VersionedCheck struct {
+	// MinimumVersion is the first policy version this check applies to.
+	// If unset, this check is not yet assigned to a policy version.
+	// If set, must not be "latest".
+	MinimumVersion api.Version
+	// CheckPod determines if the pod is allowed.
+	CheckPod CheckPodFn
+}
+
+type CheckPodFn func(*metav1.ObjectMeta, *corev1.PodSpec) CheckResult
+
+// CheckResult contains the result of checking a pod and indicates whether the pod is allowed,
+// and if not, why it was forbidden.
+//
+// Example output for (false, "host ports", "8080, 9090"):
+//   When checking all pods in a namespace:
+//     disallowed by policy "baseline": host ports, privileged containers, non-default capabilities
+//   When checking an individual pod:
+//     disallowed by policy "baseline": host ports (8080, 9090), privileged containers, non-default capabilities (CAP_NET_RAW)
+type CheckResult struct {
+	// Allowed indicates if the check allowed the pod.
+	Allowed bool
+	// ForbiddenReason must be set if Allowed is false.
+	// ForbiddenReason should be as succinct as possible and is always output.
+	// Examples:
+	// - "host ports"
+	// - "privileged containers"
+	// - "non-default capabilities"
+	ForbiddenReason string
+	// ForbiddenDetail should only be set if Allowed is false, and is optional.
+	// ForbiddenDetail can include specific values that were disallowed and is used when checking an individual object.
+	// Examples:
+	// - list specific invalid host ports: "8080, 9090"
+	// - list specific invalid containers: "container1, container2"
+	// - list specific non-default capabilities: "CAP_NET_RAW"
+	ForbiddenDetail string
+}
+
+// AggergateCheckResult holds the aggregate result of running CheckPod across multiple checks.
+type AggregateCheckResult struct {
+	// Allowed indicates if all checks allowed the pod.
+	Allowed bool
+	// ForbiddenReasons is a slice of the forbidden reasons from all the forbidden checks. It should not include empty strings.
+	// ForbiddenReasons and ForbiddenDetails must have the same number of elements, and the indexes are for the same check.
+	ForbiddenReasons []string
+	// ForbiddenDetails is a slice of the forbidden details from all the forbidden checks. It may include empty strings.
+	// ForbiddenReasons and ForbiddenDetails must have the same number of elements, and the indexes are for the same check.
+	ForbiddenDetails []string
+}
+
+// ForbiddenReason returns a comma-separated string of of the forbidden reasons.
+// Example: host ports, privileged containers, non-default capabilities
+func (a *AggregateCheckResult) ForbiddenReason() string {
+	return strings.Join(a.ForbiddenReasons, ", ")
+}
+
+// ForbiddenDetail returns a detailed forbidden message, with non-empty details formatted in
+// parentheses with the associated reason.
+// Example: host ports (8080, 9090), privileged containers, non-default capabilities (NET_RAW)
+func (a *AggregateCheckResult) ForbiddenDetail() string {
+	var b strings.Builder
+	for i := 0; i < len(a.ForbiddenReasons); i++ {
+		b.WriteString(a.ForbiddenReasons[i])
+		if a.ForbiddenDetails[i] != "" {
+			b.WriteString(" (")
+			b.WriteString(a.ForbiddenDetails[i])
+			b.WriteString(")")
+		}
+		if i != len(a.ForbiddenReasons)-1 {
+			b.WriteString(", ")
+		}
+	}
+	return b.String()
+}
+
+// UnknownForbiddenReason is used as the placeholder forbidden reason for checks that incorrectly disallow without providing a reason.
+const UnknownForbiddenReason = "unknown forbidden reason"
+
+// AggregateCheckPod runs all the checks and aggregates the forbidden results into a single CheckResult.
+// The aggregated reason is a comma-separated
+func AggregateCheckResults(results []CheckResult) AggregateCheckResult {
+	var (
+		reasons []string
+		details []string
+	)
+	for _, result := range results {
+		if !result.Allowed {
+			if len(result.ForbiddenReason) == 0 {
+				reasons = append(reasons, UnknownForbiddenReason)
+			} else {
+				reasons = append(reasons, result.ForbiddenReason)
+			}
+			details = append(details, result.ForbiddenDetail)
+		}
+	}
+	return AggregateCheckResult{
+		Allowed:          len(reasons) == 0,
+		ForbiddenReasons: reasons,
+		ForbiddenDetails: details,
+	}
+}
+
+var (
+	defaultChecks      []func() Check
+	experimentalChecks []func() Check
+)
+
+func addCheck(f func() Check) {
+	// add to experimental or versioned list
+	c := f()
+	if len(c.Versions) == 1 && c.Versions[0].MinimumVersion == (api.Version{}) {
+		experimentalChecks = append(experimentalChecks, f)
+	} else {
+		defaultChecks = append(defaultChecks, f)
+	}
+}
+
+// DefaultChecks returns checks that are expected to be enabled by default.
+// The results are mutually exclusive with ExperimentalChecks.
+// It returns a new copy of checks on each invocation and is expected to be called once at setup time.
+func DefaultChecks() []Check {
+	retval := make([]Check, 0, len(defaultChecks))
+	for _, f := range defaultChecks {
+		retval = append(retval, f())
+	}
+	return retval
+}
+
+// ExperimentalChecks returns checks that have not yet been assigned to policy versions.
+// The results are mutually exclusive with DefaultChecks.
+// It returns a new copy of checks on each invocation and is expected to be called once at setup time.
+func ExperimentalChecks() []Check {
+	retval := make([]Check, 0, len(experimentalChecks))
+	for _, f := range experimentalChecks {
+		retval = append(retval, f())
+	}
+	return retval
+}
diff --git a/staging/src/k8s.io/pod-security-admission/policy/doc.go b/staging/src/k8s.io/pod-security-admission/policy/doc.go
new file mode 100644
index 00000000000..a6211eb5dd6
--- /dev/null
+++ b/staging/src/k8s.io/pod-security-admission/policy/doc.go
@@ -0,0 +1,18 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package policy contains implementations of Pod Security Standards checks
+package policy // import "k8s.io/pod-security-admission/policy"
diff --git a/staging/src/k8s.io/pod-security-admission/policy/registry.go b/staging/src/k8s.io/pod-security-admission/policy/registry.go
new file mode 100644
index 00000000000..244bfe3787b
--- /dev/null
+++ b/staging/src/k8s.io/pod-security-admission/policy/registry.go
@@ -0,0 +1,146 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package policy
+
+import (
+	"fmt"
+
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/pod-security-admission/api"
+)
+
+// Evaluator holds the Checks that are used to validate a policy.
+type Evaluator interface {
+	// EvaluatePod evaluates the pod against the policy for the given level & version.
+	EvaluatePod(lv api.LevelVersion, podMetadata *metav1.ObjectMeta, podSpec *corev1.PodSpec) []CheckResult
+}
+
+// checkRegistry provides a default implementation of an Evaluator.
+type checkRegistry struct {
+	// The checks are a map of check_ID -> sorted slice of versioned checks, newest first
+	baselineChecks, restrictedChecks map[api.Version][]CheckPodFn
+	// maxVersion is the maximum version that is cached, guaranteed to be at least
+	// the max MinimumVersion of all registered checks.
+	maxVersion api.Version
+}
+
+// NewEvaluator constructs a new Evaluator instance from the list of checks. If the provided checks are invalid,
+// an error is returned. A valid list of checks must meet the following requirements:
+// 1. Check.ID is unique in the list
+// 2. Check.Level must be either Baseline or Restricted
+// 3. Checks must have a non-empty set of versions, sorted in a strictly increasing order
+// 4. Check.Versions cannot include 'latest'
+func NewEvaluator(checks []Check) (Evaluator, error) {
+	if err := validateChecks(checks); err != nil {
+		return nil, err
+	}
+	r := &checkRegistry{
+		baselineChecks:   map[api.Version][]CheckPodFn{},
+		restrictedChecks: map[api.Version][]CheckPodFn{},
+	}
+	populate(r, checks)
+	return r, nil
+}
+
+func (r *checkRegistry) EvaluatePod(lv api.LevelVersion, podMetadata *metav1.ObjectMeta, podSpec *corev1.PodSpec) []CheckResult {
+	if lv.Level == api.LevelPrivileged {
+		return nil
+	}
+	if r.maxVersion.Older(lv.Version) {
+		lv.Version = r.maxVersion
+	}
+	results := []CheckResult{}
+	for _, check := range r.baselineChecks[lv.Version] {
+		results = append(results, check(podMetadata, podSpec))
+	}
+	if lv.Level == api.LevelBaseline {
+		return results
+	}
+	for _, check := range r.restrictedChecks[lv.Version] {
+		results = append(results, check(podMetadata, podSpec))
+	}
+	return results
+}
+
+func validateChecks(checks []Check) error {
+	ids := map[string]bool{}
+	for _, check := range checks {
+		if ids[check.ID] {
+			return fmt.Errorf("multiple checks registered for ID %s", check.ID)
+		}
+		ids[check.ID] = true
+		if check.Level != api.LevelBaseline && check.Level != api.LevelRestricted {
+			return fmt.Errorf("check %s: invalid level %s", check.ID, check.Level)
+		}
+		if len(check.Versions) == 0 {
+			return fmt.Errorf("check %s: empty", check.ID)
+		}
+		maxVersion := api.Version{}
+		for _, c := range check.Versions {
+			if c.MinimumVersion == (api.Version{}) {
+				return fmt.Errorf("check %s: undefined version found", check.ID)
+			}
+			if c.MinimumVersion.Latest() {
+				return fmt.Errorf("check %s: version cannot be 'latest'", check.ID)
+			}
+			if maxVersion == c.MinimumVersion {
+				return fmt.Errorf("check %s: duplicate version %s", check.ID, c.MinimumVersion)
+			}
+			if !maxVersion.Older(c.MinimumVersion) {
+				return fmt.Errorf("check %s: versions must be strictly increasing", check.ID)
+			}
+			maxVersion = c.MinimumVersion
+		}
+	}
+	return nil
+}
+
+func populate(r *checkRegistry, validChecks []Check) {
+	// Find the max(MinimumVersion) across all checks.
+	for _, c := range validChecks {
+		lastVersion := c.Versions[len(c.Versions)-1].MinimumVersion
+		if r.maxVersion.Older(lastVersion) {
+			r.maxVersion = lastVersion
+		}
+	}
+
+	for _, c := range validChecks {
+		if c.Level == api.LevelRestricted {
+			inflateVersions(c, r.restrictedChecks, r.maxVersion)
+		} else {
+			inflateVersions(c, r.baselineChecks, r.maxVersion)
+		}
+	}
+}
+
+func inflateVersions(check Check, versions map[api.Version][]CheckPodFn, maxVersion api.Version) {
+	for i, c := range check.Versions {
+		var nextVersion api.Version
+		if i+1 < len(check.Versions) {
+			nextVersion = check.Versions[i+1].MinimumVersion
+		} else {
+			// Assumes only 1 Major version.
+			nextVersion = api.MajorMinorVersion(1, maxVersion.Minor()+1)
+		}
+		// Iterate over all versions from the minimum of the current check, to the minimum of the
+		// next check, or the maxVersion++.
+		for v := c.MinimumVersion; v.Older(nextVersion); v = api.MajorMinorVersion(1, v.Minor()+1) {
+			versions[v] = append(versions[v], check.Versions[i].CheckPod)
+		}
+	}
+}
diff --git a/staging/src/k8s.io/pod-security-admission/policy/registry_test.go b/staging/src/k8s.io/pod-security-admission/policy/registry_test.go
new file mode 100644
index 00000000000..cfcb15246d8
--- /dev/null
+++ b/staging/src/k8s.io/pod-security-admission/policy/registry_test.go
@@ -0,0 +1,103 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package policy
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/pod-security-admission/api"
+)
+
+func TestCheckRegistry(t *testing.T) {
+	checks := []Check{
+		generateCheck("a", api.LevelBaseline, []string{"v1.0"}),
+		generateCheck("b", api.LevelBaseline, []string{"v1.10"}),
+		generateCheck("c", api.LevelBaseline, []string{"v1.0", "v1.5", "v1.10"}),
+		generateCheck("d", api.LevelBaseline, []string{"v1.11", "v1.15", "v1.20"}),
+		generateCheck("e", api.LevelRestricted, []string{"v1.0"}),
+		generateCheck("f", api.LevelRestricted, []string{"v1.12", "v1.16", "v1.21"}),
+	}
+
+	reg, err := NewEvaluator(checks)
+	require.NoError(t, err)
+
+	levelCases := []struct {
+		level           api.Level
+		version         string
+		expectedReasons []string
+	}{
+		{api.LevelPrivileged, "v1.0", nil},
+		{api.LevelPrivileged, "latest", nil},
+		{api.LevelBaseline, "v1.0", []string{"a:v1.0", "c:v1.0"}},
+		{api.LevelBaseline, "v1.4", []string{"a:v1.0", "c:v1.0"}},
+		{api.LevelBaseline, "v1.5", []string{"a:v1.0", "c:v1.5"}},
+		{api.LevelBaseline, "v1.10", []string{"a:v1.0", "b:v1.10", "c:v1.10"}},
+		{api.LevelBaseline, "v1.11", []string{"a:v1.0", "b:v1.10", "c:v1.10", "d:v1.11"}},
+		{api.LevelBaseline, "latest", []string{"a:v1.0", "b:v1.10", "c:v1.10", "d:v1.20"}},
+		{api.LevelRestricted, "v1.0", []string{"a:v1.0", "c:v1.0", "e:v1.0"}},
+		{api.LevelRestricted, "v1.4", []string{"a:v1.0", "c:v1.0", "e:v1.0"}},
+		{api.LevelRestricted, "v1.5", []string{"a:v1.0", "c:v1.5", "e:v1.0"}},
+		{api.LevelRestricted, "v1.10", []string{"a:v1.0", "b:v1.10", "c:v1.10", "e:v1.0"}},
+		{api.LevelRestricted, "v1.11", []string{"a:v1.0", "b:v1.10", "c:v1.10", "d:v1.11", "e:v1.0"}},
+		{api.LevelRestricted, "latest", []string{"a:v1.0", "b:v1.10", "c:v1.10", "d:v1.20", "e:v1.0", "f:v1.21"}},
+		{api.LevelRestricted, "v1.10000", []string{"a:v1.0", "b:v1.10", "c:v1.10", "d:v1.20", "e:v1.0", "f:v1.21"}},
+	}
+	for _, test := range levelCases {
+		t.Run(fmt.Sprintf("%s:%s", test.level, test.version), func(t *testing.T) {
+			results := reg.EvaluatePod(api.LevelVersion{test.level, versionOrPanic(test.version)}, nil, nil)
+
+			// Set extract the ForbiddenReasons from the results.
+			var actualReasons []string
+			for _, result := range results {
+				actualReasons = append(actualReasons, result.ForbiddenReason)
+			}
+			assert.ElementsMatch(t, test.expectedReasons, actualReasons)
+		})
+	}
+}
+
+func generateCheck(id string, level api.Level, versions []string) Check {
+	c := Check{
+		ID:    id,
+		Level: level,
+	}
+	for _, ver := range versions {
+		v := versionOrPanic(ver) // Copy ver so it can be used in the CheckPod closure.
+		c.Versions = append(c.Versions, VersionedCheck{
+			MinimumVersion: v,
+			CheckPod: func(_ *metav1.ObjectMeta, _ *corev1.PodSpec) CheckResult {
+				return CheckResult{
+					ForbiddenReason: fmt.Sprintf("%s:%s", id, v),
+				}
+			},
+		})
+	}
+	return c
+}
+
+func versionOrPanic(v string) api.Version {
+	ver, err := api.ParseVersion(v)
+	if err != nil {
+		panic(err)
+	}
+	return ver
+}
diff --git a/staging/src/k8s.io/pod-security-admission/policy/visitor.go b/staging/src/k8s.io/pod-security-admission/policy/visitor.go
new file mode 100644
index 00000000000..d8e3cbe0e9e
--- /dev/null
+++ b/staging/src/k8s.io/pod-security-admission/policy/visitor.go
@@ -0,0 +1,42 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package policy
+
+import (
+	corev1 "k8s.io/api/core/v1"
+	"k8s.io/apimachinery/pkg/util/validation/field"
+)
+
+// ContainerVisitorWithPath is called with each container and the field.Path to that container
+type ContainerVisitorWithPath func(container *corev1.Container, path *field.Path)
+
+// visitContainersWithPath invokes the visitor function with a pointer to the spec
+// of every container in the given pod spec and the field.Path to that container.
+func visitContainersWithPath(podSpec *corev1.PodSpec, specPath *field.Path, visitor ContainerVisitorWithPath) {
+	fldPath := specPath.Child("initContainers")
+	for i := range podSpec.InitContainers {
+		visitor(&podSpec.InitContainers[i], fldPath.Index(i))
+	}
+	fldPath = specPath.Child("containers")
+	for i := range podSpec.Containers {
+		visitor(&podSpec.Containers[i], fldPath.Index(i))
+	}
+	fldPath = specPath.Child("ephemeralContainers")
+	for i := range podSpec.EphemeralContainers {
+		visitor((*corev1.Container)(&podSpec.EphemeralContainers[i].EphemeralContainerCommon), fldPath.Index(i))
+	}
+}