diff --git a/core/src/main/kotlin/com/r3corda/core/Utils.kt b/core/src/main/kotlin/com/r3corda/core/Utils.kt
index 4598e15a2a..ba6edf8d29 100644
--- a/core/src/main/kotlin/com/r3corda/core/Utils.kt
+++ b/core/src/main/kotlin/com/r3corda/core/Utils.kt
@@ -142,7 +142,7 @@ inline fun <T> logElapsedTime(label: String, logger: Logger? = null, body: () ->
  *
  * val ii = state.locked { i }
  */
-class ThreadBox<T>(content: T, val lock: ReentrantLock = ReentrantLock()) {
+class ThreadBox<out T>(content: T, val lock: ReentrantLock = ReentrantLock()) {
     val content = content
     inline fun <R> locked(body: T.() -> R): R = lock.withLock { body(content) }
     inline fun <R> alreadyLocked(body: T.() -> R): R {
@@ -164,7 +164,7 @@ abstract class RetryableException(message: String) : Exception(message)
  * will not be serialized to disk, and if it's missing (or the first time it's accessed), the initializer will be
  * used to set it up. Note that the initializer will be called with the TransientProperty object locked.
  */
-class TransientProperty<T>(private val initializer: () -> T) {
+class TransientProperty<out T>(private val initializer: () -> T) {
     @Transient private var v: T? = null
 
     @Synchronized
diff --git a/core/src/main/kotlin/com/r3corda/core/contracts/Structures.kt b/core/src/main/kotlin/com/r3corda/core/contracts/Structures.kt
index 774f14487b..28b293512b 100644
--- a/core/src/main/kotlin/com/r3corda/core/contracts/Structures.kt
+++ b/core/src/main/kotlin/com/r3corda/core/contracts/Structures.kt
@@ -132,7 +132,7 @@ interface IssuanceDefinition
  *
  * @param P the type of product underlying the definition, for example [Currency].
  */
-data class Issued<P>(
+data class Issued<out P>(
         val issuer: PartyAndReference,
         val product: P
 )
diff --git a/core/src/main/kotlin/com/r3corda/core/contracts/clauses/GroupClauseVerifier.kt b/core/src/main/kotlin/com/r3corda/core/contracts/clauses/GroupClauseVerifier.kt
index 38bcf37262..2cdaf61f2b 100644
--- a/core/src/main/kotlin/com/r3corda/core/contracts/clauses/GroupClauseVerifier.kt
+++ b/core/src/main/kotlin/com/r3corda/core/contracts/clauses/GroupClauseVerifier.kt
@@ -6,7 +6,7 @@ import com.r3corda.core.contracts.ContractState
 import com.r3corda.core.contracts.TransactionForContract
 import java.util.*
 
-interface GroupVerify<S, T : Any> {
+interface GroupVerify<in S, in T : Any> {
     /**
      *
      * @return the set of commands that are consumed IF this clause is matched, and cannot be used to match a
diff --git a/core/src/main/kotlin/com/r3corda/core/protocols/ProtocolLogic.kt b/core/src/main/kotlin/com/r3corda/core/protocols/ProtocolLogic.kt
index c07cf36d86..5a79a4f4ca 100644
--- a/core/src/main/kotlin/com/r3corda/core/protocols/ProtocolLogic.kt
+++ b/core/src/main/kotlin/com/r3corda/core/protocols/ProtocolLogic.kt
@@ -24,7 +24,7 @@ import org.slf4j.Logger
  * If you'd like to use another ProtocolLogic class as a component of your own, construct it on the fly and then pass
  * it to the [subProtocol] method. It will return the result of that protocol when it completes.
  */
-abstract class ProtocolLogic<T> {
+abstract class ProtocolLogic<out T> {
 
     /** Reference to the [Fiber] instance that is the top level controller for the entire flow. */
     lateinit var psm: ProtocolStateMachine<*>
diff --git a/core/src/main/kotlin/com/r3corda/core/utilities/NonEmptySet.kt b/core/src/main/kotlin/com/r3corda/core/utilities/NonEmptySet.kt
index a12b5c49c7..1e961f7539 100644
--- a/core/src/main/kotlin/com/r3corda/core/utilities/NonEmptySet.kt
+++ b/core/src/main/kotlin/com/r3corda/core/utilities/NonEmptySet.kt
@@ -72,7 +72,7 @@ class NonEmptySet<T>(initial: T) : MutableSet<T> {
     override fun hashCode(): Int = set.hashCode()
     override fun toString(): String = set.toString()
 
-    inner class Iterator<T>(val iterator: MutableIterator<T>) : MutableIterator<T> {
+    inner class Iterator<out T>(val iterator: MutableIterator<T>) : MutableIterator<T> {
         override fun hasNext(): Boolean = iterator.hasNext()
         override fun next(): T = iterator.next()
         override fun remove() =
diff --git a/core/src/main/kotlin/com/r3corda/core/utilities/RecordingMap.kt b/core/src/main/kotlin/com/r3corda/core/utilities/RecordingMap.kt
index 4688518656..35f5b2765c 100644
--- a/core/src/main/kotlin/com/r3corda/core/utilities/RecordingMap.kt
+++ b/core/src/main/kotlin/com/r3corda/core/utilities/RecordingMap.kt
@@ -16,8 +16,8 @@ class RecordingMap<K, V>(private val wrappedMap: MutableMap<K, V>,
     // If/when Kotlin supports data classes inside sealed classes, that would be preferable to this.
     interface Record
 
-    data class Get<K>(val key: K) : Record
-    data class Put<K, V>(val key: K, val value: V) : Record
+    data class Get<out K>(val key: K) : Record
+    data class Put<out K, out V>(val key: K, val value: V) : Record
 
     private val _records = Collections.synchronizedList(ArrayList<Record>())
 
diff --git a/core/src/main/kotlin/com/r3corda/core/utilities/UntrustworthyData.kt b/core/src/main/kotlin/com/r3corda/core/utilities/UntrustworthyData.kt
index 9b94d7722e..cb450b0ce6 100644
--- a/core/src/main/kotlin/com/r3corda/core/utilities/UntrustworthyData.kt
+++ b/core/src/main/kotlin/com/r3corda/core/utilities/UntrustworthyData.kt
@@ -11,7 +11,7 @@ package com.r3corda.core.utilities
  * - Are any objects *reachable* from this object mismatched or not what you expected?
  * - Is it suspiciously large or small?
  */
-class UntrustworthyData<T>(private val fromUntrustedWorld: T) {
+class UntrustworthyData<out T>(private val fromUntrustedWorld: T) {
     val data: T
         @Deprecated("Accessing the untrustworthy data directly without validating it first is a bad idea")
         get() = fromUntrustedWorld
diff --git a/core/src/main/kotlin/com/r3corda/protocols/FetchDataProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/FetchDataProtocol.kt
index 48197eb970..f2269ae491 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/FetchDataProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/FetchDataProtocol.kt
@@ -27,7 +27,7 @@ import java.util.*
  * @param T The ultimate type of the data being fetched.
  * @param W The wire type of the data being fetched, for when it isn't the same as the ultimate type.
  */
-abstract class FetchDataProtocol<T : NamedByHash, W : Any>(
+abstract class FetchDataProtocol<T : NamedByHash, in W : Any>(
         protected val requests: Set<SecureHash>,
         protected val otherSide: Party) : ProtocolLogic<FetchDataProtocol.Result<T>>() {
 
@@ -36,7 +36,7 @@ abstract class FetchDataProtocol<T : NamedByHash, W : Any>(
     class DownloadedVsRequestedDataMismatch(val requested: SecureHash, val got: SecureHash) : BadAnswer()
 
     data class Request(val hashes: List<SecureHash>, override val replyToParty: Party, override val sessionID: Long) : PartyRequestMessage
-    data class Result<T : NamedByHash>(val fromDisk: List<T>, val downloaded: List<T>)
+    data class Result<out T : NamedByHash>(val fromDisk: List<T>, val downloaded: List<T>)
 
     @Suspendable
     override fun call(): Result<T> {
diff --git a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
index b81cefa4f7..3429807fed 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
@@ -44,7 +44,7 @@ object TwoPartyDealProtocol {
     }
 
     // This object is serialised to the network and is the first protocol message the seller sends to the buyer.
-    data class Handshake<T>(
+    data class Handshake<out T>(
             val payload: T,
             val publicKey: PublicKey,
             val sessionID: Long
@@ -58,7 +58,7 @@ object TwoPartyDealProtocol {
      * There's a good chance we can push at least some of this logic down into core protocol logic
      * and helper methods etc.
      */
-    abstract class Primary<U>(override val progressTracker: ProgressTracker = Primary.tracker()) : ProtocolLogic<SignedTransaction>() {
+    abstract class Primary<out U>(override val progressTracker: ProgressTracker = Primary.tracker()) : ProtocolLogic<SignedTransaction>() {
 
         companion object {
             object AWAITING_PROPOSAL : ProgressTracker.Step("Handshaking and awaiting transaction proposal")
diff --git a/core/src/main/kotlin/protocols/AbstractStateReplacementProtocol.kt b/core/src/main/kotlin/protocols/AbstractStateReplacementProtocol.kt
index 1ae3e98408..a2a52e2d80 100644
--- a/core/src/main/kotlin/protocols/AbstractStateReplacementProtocol.kt
+++ b/core/src/main/kotlin/protocols/AbstractStateReplacementProtocol.kt
@@ -28,7 +28,7 @@ import java.security.PublicKey
  * use the new updated state for future transactions.
  */
 abstract class AbstractStateReplacementProtocol<T> {
-    interface Proposal<T> {
+    interface Proposal<out T> {
         val stateRef: StateRef
         val modification: T
         val stx: SignedTransaction