diff --git a/libraries/stdlib/src/kotlin/contracts/ContractBuilder.kt b/libraries/stdlib/src/kotlin/contracts/ContractBuilder.kt
index fb540a5ac1d..42200dc3e41 100644
--- a/libraries/stdlib/src/kotlin/contracts/ContractBuilder.kt
+++ b/libraries/stdlib/src/kotlin/contracts/ContractBuilder.kt
@@ -8,31 +8,119 @@ package kotlin.contracts
 import kotlin.internal.ContractsDsl
 import kotlin.internal.InlineOnly
 
+/**
+ * Marker of the use experimental contracts API.
+ * Any declaration annotated with that marker must be used with the [UseExperimental] annotation
+ * or the compiler argument `-Xuse-experimental=kotlin.contracts.ExperimentalContracts`.
+ */
 @Retention(AnnotationRetention.BINARY)
 @SinceKotlin("1.3")
 @Experimental
 public annotation class ExperimentalContracts
 
+/**
+ * The builder is used to specify contract effects for some function.
+ *
+ * @see contract
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public interface ContractBuilder {
+    /**
+     * Expresses that a function returned successfully.
+     *
+     * @sample samples.contracts.returnsContract
+     */
     @ContractsDsl public fun returns(): Returns
+
+    /**
+     * Expresses that a function returned with some value.
+     * Value can only be `true`, `false` or `null`.
+     *
+     * @sample samples.contracts.returnsTrueContract
+     * @sample samples.contracts.returnsFalseContract
+     * @sample samples.contracts.returnsNullContract
+     */
     @ContractsDsl public fun returns(value: Any?): Returns
+
+    /**
+     * Expresses that a function returned with any not null value.
+     *
+     * @sample samples.contracts.returnsNotNullContract
+     */
     @ContractsDsl public fun returnsNotNull(): ReturnsNotNull
+
+    /**
+     * Expresses that:
+     *  1) [lambda] will not be called after the call to owner-function is finished;
+     *  2) [lambda] will not be passed to another function without the similar contract.
+     *
+     * @param kind amount of times, that a [lambda] guaranteed will be invoked.
+     *
+     * Note that a function with the `callsInPlace` effect must be inline.
+     *
+     * @sample samples.contracts.callsInPlaceAtMostOnceContract
+     * @sample samples.contracts.callsInPlaceAtLeastOnceContract
+     * @sample samples.contracts.callsInPlaceExactlyOnceContract
+     * @sample samples.contracts.callsInPlaceUnknownContract
+     */
     @ContractsDsl public fun <R> callsInPlace(lambda: Function<R>, kind: InvocationKind = InvocationKind.UNKNOWN): CallsInPlace
 }
 
+/**
+ * This enum class is used to specify the amount of times, that `callable` which is used in the [ContractBuilder.callsInPlace] effect guaranteed will be invoked.
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public enum class InvocationKind {
+    /**
+     * Expresses that the `callable` will be invoked zero or one time.
+     *
+     * @sample samples.contracts.callsInPlaceAtMostOnceContract
+     */
     @ContractsDsl AT_MOST_ONCE,
+
+    /**
+     * Expresses that the `callable` will be invoked one or more times.
+     *
+     * @sample samples.contracts.callsInPlaceAtLeastOnceContract
+     */
     @ContractsDsl AT_LEAST_ONCE,
+
+    /**
+     * Expresses that the `callable` will be invoked exactly one time.
+     *
+     * @sample samples.contracts.callsInPlaceExactlyOnceContract
+     */
     @ContractsDsl EXACTLY_ONCE,
+
+    /**
+     * Expresses that the `callable` will be invoked unknown amount of times.
+     *
+     * @sample samples.contracts.callsInPlaceUnknownContract
+     */
     @ContractsDsl UNKNOWN
 }
 
+/**
+ * The function to describe a contract.
+ * The contract description must be at the beginning of a function and has at least one effect.
+ * Also the contract description can be used only in the top-level functions.
+ *
+ * @param builder the lambda in the body of which the effects from the [ContractBuilder] are specified.
+ *
+ * @sample samples.contracts.returnsContract
+ * @sample samples.contracts.returnsTrueContract
+ * @sample samples.contracts.returnsFalseContract
+ * @sample samples.contracts.returnsNullContract
+ * @sample samples.contracts.returnsNotNullContract
+ * @sample samples.contracts.callsInPlaceAtMostOnceContract
+ * @sample samples.contracts.callsInPlaceAtLeastOnceContract
+ * @sample samples.contracts.callsInPlaceExactlyOnceContract
+ * @sample samples.contracts.callsInPlaceUnknownContract
+ */
 @ContractsDsl
 @ExperimentalContracts
 @InlineOnly
diff --git a/libraries/stdlib/src/kotlin/contracts/Effect.kt b/libraries/stdlib/src/kotlin/contracts/Effect.kt
index 2550ad0d0d1..90269498338 100644
--- a/libraries/stdlib/src/kotlin/contracts/Effect.kt
+++ b/libraries/stdlib/src/kotlin/contracts/Effect.kt
@@ -12,31 +12,62 @@ import kotlin.internal.ContractsDsl
 @SinceKotlin("1.3")
 public interface Effect
 
+/**
+ * This interface corresponds to an additional condition applied to the [SimpleEffect].
+ * Can only be used in conjunction with [SimpleEffect].
+ *
+ * @see SimpleEffect.implies
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public interface ConditionalEffect : Effect
 
+/**
+ * Describes of function's behavior regardless of any conditions.
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public interface SimpleEffect {
+    /**
+     * The function to specify an additional condition for some `SimpleEffect`.
+     *
+     * @param booleanExpression the additional condition which is a subset of the Kotlin boolean expressions.
+     *
+     * Note: the subset of the Kotlin boolean expressions is true/false/null checks (using the equality expression) for a function parameter or receiver, also allowed the conjunction and disjunction.
+     */
     @ContractsDsl
     @ExperimentalContracts
     public infix fun implies(booleanExpression: Boolean): ConditionalEffect
 }
 
-
+/**
+ * Describes the return value of a function.
+ * The return value can only be `true`, `false` or `null`.
+ *
+ * @see ContractBuilder.returns
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public interface Returns : SimpleEffect
 
+/**
+ * Describes that the return value of a function is not null.
+ *
+ * @see ContractBuilder.returnsNotNull
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")
 public interface ReturnsNotNull : SimpleEffect
 
+/**
+ * Describes of function's behavior in relation to the invocation of the received lambda.
+ *
+ * @see ContractBuilder.callsInPlace
+ */
 @ContractsDsl
 @ExperimentalContracts
 @SinceKotlin("1.3")