diff --git a/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation.go b/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation.go
index ae893607c6b..66512edefe3 100644
--- a/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation.go
+++ b/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation.go
@@ -17,12 +17,111 @@ limitations under the License.
 package validation
 
 import (
+	"strings"
+
+	machinery "k8s.io/apimachinery/pkg/api/validation"
+	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/apimachinery/pkg/util/validation/field"
-	"k8s.io/pod-security-admission/admission/api"
+	admissionapi "k8s.io/pod-security-admission/admission/api"
+	"k8s.io/pod-security-admission/api"
 )
 
-func ValidatePodSecurityConfiguration(configuration *api.PodSecurityConfiguration) field.ErrorList {
+// ValidatePodSecurityConfiguration validates a given PodSecurityConfiguration.
+func ValidatePodSecurityConfiguration(configuration *admissionapi.PodSecurityConfiguration) field.ErrorList {
 	allErrs := field.ErrorList{}
-	// TODO: validate default levels and versions
+
+	// validate defaults
+	allErrs = append(allErrs, validateLevel(field.NewPath("defaults", "enforce"), configuration.Defaults.Enforce)...)
+	allErrs = append(allErrs, validateVersion(field.NewPath("defaults", "enforce-version"), configuration.Defaults.EnforceVersion)...)
+	allErrs = append(allErrs, validateLevel(field.NewPath("defaults", "warn"), configuration.Defaults.Warn)...)
+	allErrs = append(allErrs, validateVersion(field.NewPath("defaults", "warn-version"), configuration.Defaults.WarnVersion)...)
+	allErrs = append(allErrs, validateLevel(field.NewPath("defaults", "audit"), configuration.Defaults.Audit)...)
+	allErrs = append(allErrs, validateVersion(field.NewPath("defaults", "audit-version"), configuration.Defaults.AuditVersion)...)
+
+	// validate exemptions
+	allErrs = append(allErrs, validateNamespaces(configuration)...)
+	allErrs = append(allErrs, validateRuntimeClasses(configuration)...)
+	allErrs = append(allErrs, validateUsernames(configuration)...)
+
 	return allErrs
 }
+
+// validateLevel validates a level
+func validateLevel(p *field.Path, value string) field.ErrorList {
+	errs := field.ErrorList{}
+	_, err := api.ParseLevel(value)
+	if err != nil {
+		errs = append(errs, field.Invalid(p, value, err.Error()))
+	}
+	return errs
+}
+
+// validateVersion validates a version
+func validateVersion(p *field.Path, value string) field.ErrorList {
+	errs := field.ErrorList{}
+	_, err := api.ParseVersion(value)
+	if err != nil {
+		errs = append(errs, field.Invalid(p, value, err.Error()))
+	}
+	return errs
+}
+
+func validateNamespaces(configuration *admissionapi.PodSecurityConfiguration) field.ErrorList {
+	errs := field.ErrorList{}
+	validSet := sets.NewString()
+	for i, ns := range configuration.Exemptions.Namespaces {
+		err := machinery.ValidateNamespaceName(ns, false)
+		if len(err) > 0 {
+			path := field.NewPath("exemptions", "namespaces").Index(i)
+			errs = append(errs, field.Invalid(path, ns, strings.Join(err, ", ")))
+			continue
+		}
+		if validSet.Has(ns) {
+			path := field.NewPath("exemptions", "namespaces").Index(i)
+			errs = append(errs, field.Duplicate(path, ns))
+			continue
+		}
+		validSet.Insert(ns)
+	}
+	return errs
+}
+
+func validateRuntimeClasses(configuration *admissionapi.PodSecurityConfiguration) field.ErrorList {
+	errs := field.ErrorList{}
+	validSet := sets.NewString()
+	for i, rc := range configuration.Exemptions.RuntimeClasses {
+		err := machinery.NameIsDNSSubdomain(rc, false)
+		if len(err) > 0 {
+			path := field.NewPath("exemptions", "runtimeClasses").Index(i)
+			errs = append(errs, field.Invalid(path, rc, strings.Join(err, ", ")))
+			continue
+		}
+		if validSet.Has(rc) {
+			path := field.NewPath("exemptions", "runtimeClasses").Index(i)
+			errs = append(errs, field.Duplicate(path, rc))
+			continue
+		}
+		validSet.Insert(rc)
+	}
+	return errs
+}
+
+func validateUsernames(configuration *admissionapi.PodSecurityConfiguration) field.ErrorList {
+	errs := field.ErrorList{}
+	validSet := sets.NewString()
+	for i, uname := range configuration.Exemptions.Usernames {
+		if uname == "" {
+			path := field.NewPath("exemptions", "usernames").Index(i)
+			errs = append(errs, field.Invalid(path, uname, "username must not be empty"))
+			continue
+		}
+		if validSet.Has(uname) {
+			path := field.NewPath("exemptions", "usernames").Index(i)
+			errs = append(errs, field.Duplicate(path, uname))
+			continue
+		}
+		validSet.Insert(uname)
+	}
+
+	return errs
+}
diff --git a/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation_test.go b/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation_test.go
index 10aebbd6245..df8a17446d9 100644
--- a/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation_test.go
+++ b/staging/src/k8s.io/pod-security-admission/admission/api/validation/validation_test.go
@@ -15,3 +15,129 @@ limitations under the License.
 */
 
 package validation
+
+import (
+	"testing"
+
+	"k8s.io/apimachinery/pkg/util/validation/field"
+	"k8s.io/pod-security-admission/admission/api"
+)
+
+type (
+	test struct {
+		configuration   api.PodSecurityConfiguration
+		expectedErrList field.ErrorList
+	}
+)
+
+const (
+	invalidValueUppercase = "TEST"
+	invalidValueChars     = "_&$%#"
+	invalidValueTooLong   = "testtesttesttesttesttesttesttesttesttesttesttesttesttesttesttest"
+	invalidValueEmpty     = ""
+	validValue            = "testing"
+)
+
+func TestValidatePodSecurityConfiguration(t *testing.T) {
+	tests := []test{
+		// defaults
+		{
+			expectedErrList: field.ErrorList{
+				field.Invalid(exemptionsPath("namespaces", 0), invalidValueEmpty, "..."),
+				field.Invalid(exemptionsPath("namespaces", 1), invalidValueChars, "..."),
+				field.Invalid(exemptionsPath("namespaces", 2), invalidValueUppercase, "..."),
+				field.Invalid(exemptionsPath("namespaces", 3), invalidValueTooLong, "..."),
+				field.Duplicate(exemptionsPath("namespaces", 5), validValue),
+				field.Invalid(exemptionsPath("runtimeClasses", 0), invalidValueEmpty, "..."),
+				field.Invalid(exemptionsPath("runtimeClasses", 1), invalidValueChars, "..."),
+				field.Invalid(exemptionsPath("runtimeClasses", 2), invalidValueUppercase, "..."),
+				field.Duplicate(exemptionsPath("runtimeClasses", 4), validValue),
+				field.Invalid(exemptionsPath("usernames", 0), invalidValueEmpty, "..."),
+				field.Duplicate(exemptionsPath("usernames", 2), validValue),
+			},
+			configuration: api.PodSecurityConfiguration{
+				Defaults: api.PodSecurityDefaults{
+					Enforce:        "privileged",
+					EnforceVersion: "v1.22",
+					Audit:          "baseline",
+					AuditVersion:   "v1.25",
+					Warn:           "restricted",
+					WarnVersion:    "latest",
+				},
+				Exemptions: api.PodSecurityExemptions{
+					Namespaces: []string{
+						invalidValueEmpty,
+						invalidValueChars,
+						invalidValueUppercase,
+						invalidValueTooLong,
+						validValue,
+						validValue,
+					},
+					RuntimeClasses: []string{
+						invalidValueEmpty,
+						invalidValueChars,
+						invalidValueUppercase,
+						validValue,
+						validValue,
+					},
+					Usernames: []string{
+						invalidValueEmpty,
+						validValue,
+						validValue,
+					},
+				},
+			},
+		},
+		{
+			expectedErrList: field.ErrorList{
+				field.Invalid(defaultsPath("enforce"), "baslein", "..."),
+				field.Invalid(defaultsPath("enforce-version"), "v.122", "..."),
+				field.Invalid(defaultsPath("warn"), "", "..."),
+				field.Invalid(defaultsPath("warn-version"), "", "..."),
+				field.Invalid(defaultsPath("audit"), "lorum", "..."),
+				field.Invalid(defaultsPath("audit-version"), "ipsum", "..."),
+			},
+			configuration: api.PodSecurityConfiguration{
+				Defaults: api.PodSecurityDefaults{
+					Enforce:        "baslein",
+					EnforceVersion: "v.122",
+					Audit:          "lorum",
+					AuditVersion:   "ipsum",
+					Warn:           "",
+					WarnVersion:    "",
+				},
+				Exemptions: api.PodSecurityExemptions{},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		errList := ValidatePodSecurityConfiguration(&test.configuration)
+		if len(errList) != len(test.expectedErrList) {
+			t.Errorf("expected %d errs, got %d", len(test.expectedErrList), len(errList))
+		}
+
+		for i, expected := range test.expectedErrList {
+			if expected.Type.String() != errList[i].Type.String() {
+				t.Errorf("expected err type %s, got %s",
+					expected.Type.String(),
+					errList[i].Type.String())
+			}
+			if expected.BadValue != errList[i].BadValue {
+				t.Errorf("expected bad value '%s', got '%s'",
+					expected.BadValue,
+					errList[i].BadValue)
+			}
+		}
+	}
+}
+
+// defaultsPath returns the appropriate defaults path
+func defaultsPath(child string) *field.Path {
+	return field.NewPath("defaults", child)
+}
+
+// exemptionsPath returns the appropriate defaults path
+func exemptionsPath(child string, i int) *field.Path {
+	return field.NewPath("exemptions", child).Index(i)
+}