diff --git a/solidity/apps/contracts/governance/GovernanceRouter.sol b/solidity/apps/contracts/governance/GovernanceRouter.sol
index 42d53bf28..d18f98c5f 100644
--- a/solidity/apps/contracts/governance/GovernanceRouter.sol
+++ b/solidity/apps/contracts/governance/GovernanceRouter.sol
@@ -7,7 +7,7 @@ import {GovernanceMessage} from "./GovernanceMessage.sol";
 // ============ External Imports ============
 import {Version0} from "@abacus-network/core/contracts/Version0.sol";
 import {Router} from "@abacus-network/core/contracts/router/Router.sol";
-import {TypeCasts} from "@abacus-network/core/contracts/XAppConnectionManager.sol";
+import {TypeCasts} from "@abacus-network/core/libs/TypeCasts.sol";
 import {Initializable} from "@openzeppelin/contracts/proxy/Initializable.sol";
 import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";
 import {TypedMemView} from "@summa-tx/memview-sol/contracts/TypedMemView.sol";
diff --git a/solidity/apps/contracts/ping-pong/PingPongRouter.sol b/solidity/apps/contracts/ping-pong/PingPongRouter.sol
index 1da3f5e2a..4e5b24090 100644
--- a/solidity/apps/contracts/ping-pong/PingPongRouter.sol
+++ b/solidity/apps/contracts/ping-pong/PingPongRouter.sol
@@ -154,11 +154,7 @@ contract PingPongRouter is Router {
             ? PingPongMessage.formatPing(_match, _count)
             : PingPongMessage.formatPong(_match, _count);
         // send the message to the xApp Router
-        (_outbox()).dispatch(
-            _destinationDomain,
-            _remoteRouterAddress,
-            _message
-        );
+        _outbox().dispatch(_destinationDomain, _remoteRouterAddress, _message);
         // emit a Sent event
         emit Sent(_destinationDomain, _match, _count, _isPing);
     }
diff --git a/solidity/core/contracts/Common.sol b/solidity/core/contracts/Common.sol
index 46857504d..ba13d624d 100644
--- a/solidity/core/contracts/Common.sol
+++ b/solidity/core/contracts/Common.sol
@@ -3,6 +3,7 @@ pragma solidity >=0.6.11;
 
 // ============ Internal Imports ============
 import {IValidatorManager} from "../interfaces/IValidatorManager.sol";
+import {ICommon} from "../interfaces/ICommon.sol";
 // ============ External Imports ============
 import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
 import {Address} from "@openzeppelin/contracts/utils/Address.sol";
@@ -12,11 +13,11 @@ import {Address} from "@openzeppelin/contracts/utils/Address.sol";
  * @author Celo Labs Inc.
  * @notice Shared utilities between Outbox and Inbox.
  */
-abstract contract Common is OwnableUpgradeable {
+abstract contract Common is ICommon, OwnableUpgradeable {
     // ============ Immutable Variables ============
 
     // Domain of chain on which the contract is deployed
-    uint32 public immutable localDomain;
+    uint32 public immutable override localDomain;
 
     // ============ Public Variables ============
 
@@ -88,6 +89,7 @@ abstract contract Common is OwnableUpgradeable {
     function latestCheckpoint()
         external
         view
+        override
         returns (bytes32 root, uint256 index)
     {
         root = checkpointedRoot;
diff --git a/solidity/core/contracts/Inbox.sol b/solidity/core/contracts/Inbox.sol
index 46633e197..181fae568 100644
--- a/solidity/core/contracts/Inbox.sol
+++ b/solidity/core/contracts/Inbox.sol
@@ -7,6 +7,7 @@ import {Common} from "./Common.sol";
 import {MerkleLib} from "../libs/Merkle.sol";
 import {Message} from "../libs/Message.sol";
 import {IMessageRecipient} from "../interfaces/IMessageRecipient.sol";
+import {IInbox} from "../interfaces/IInbox.sol";
 // ============ External Imports ============
 import {TypedMemView} from "@summa-tx/memview-sol/contracts/TypedMemView.sol";
 
@@ -16,7 +17,7 @@ import {TypedMemView} from "@summa-tx/memview-sol/contracts/TypedMemView.sol";
  * @notice Track root updates on Outbox, prove and dispatch messages to end
  * recipients.
  */
-contract Inbox is Version0, Common {
+contract Inbox is IInbox, Version0, Common {
     // ============ Libraries ============
 
     using MerkleLib for MerkleLib.Tree;
@@ -39,7 +40,7 @@ contract Inbox is Version0, Common {
     // ============ Public Storage ============
 
     // Domain of outbox chain
-    uint32 public remoteDomain;
+    uint32 public override remoteDomain;
     // re-entrancy guard
     uint8 private entered;
     // Mapping of message leaves to MessageStatus
@@ -90,7 +91,7 @@ contract Inbox is Version0, Common {
         bytes32 _root,
         uint256 _index,
         bytes calldata _signature
-    ) external {
+    ) external override {
         // ensure that update is more recent than the latest we've seen
         require(_index > checkpoints[checkpointedRoot], "old checkpoint");
         // validate validator signature
@@ -119,7 +120,7 @@ contract Inbox is Version0, Common {
         bytes calldata _message,
         bytes32[32] calldata _proof,
         uint256 _index
-    ) external {
+    ) external override {
         require(prove(keccak256(_message), _proof, _index), "!prove");
         process(_message);
     }
diff --git a/solidity/core/contracts/Outbox.sol b/solidity/core/contracts/Outbox.sol
index a234a9bfa..748d10d35 100644
--- a/solidity/core/contracts/Outbox.sol
+++ b/solidity/core/contracts/Outbox.sol
@@ -7,6 +7,7 @@ import {Common} from "./Common.sol";
 import {MerkleLib} from "../libs/Merkle.sol";
 import {Message} from "../libs/Message.sol";
 import {MerkleTreeManager} from "./Merkle.sol";
+import {IOutbox} from "../interfaces/IOutbox.sol";
 
 /**
  * @title Outbox
@@ -18,7 +19,7 @@ import {MerkleTreeManager} from "./Merkle.sol";
  * Accepts submissions of fraudulent signatures
  * by the Validator and slashes the Validator in this case.
  */
-contract Outbox is Version0, MerkleTreeManager, Common {
+contract Outbox is IOutbox, Version0, MerkleTreeManager, Common {
     // ============ Libraries ============
 
     using MerkleLib for MerkleLib.Tree;
@@ -121,7 +122,7 @@ contract Outbox is Version0, MerkleTreeManager, Common {
         uint32 _destinationDomain,
         bytes32 _recipientAddress,
         bytes memory _messageBody
-    ) external notFailed returns (uint256) {
+    ) external override notFailed returns (uint256) {
         require(_messageBody.length <= MAX_MESSAGE_BODY_BYTES, "msg too long");
         // get the next nonce for the destination domain, then increment it
         uint32 _nonce = nonces[_destinationDomain];
@@ -157,7 +158,7 @@ contract Outbox is Version0, MerkleTreeManager, Common {
      * relayed to the Inbox contracts.
      * @dev emits Checkpoint event
      */
-    function checkpoint() external notFailed {
+    function checkpoint() external override notFailed {
         uint256 count = count();
         require(count > 0, "!count");
         bytes32 root = root();
diff --git a/solidity/core/contracts/XAppConnectionManager.sol b/solidity/core/contracts/XAppConnectionManager.sol
index befa2247f..f71d71cf1 100644
--- a/solidity/core/contracts/XAppConnectionManager.sol
+++ b/solidity/core/contracts/XAppConnectionManager.sol
@@ -2,12 +2,9 @@
 pragma solidity >=0.6.11;
 
 // ============ Internal Imports ============
-import {Outbox} from "./Outbox.sol";
-import {Inbox} from "./Inbox.sol";
+import {IOutbox} from "../interfaces/IOutbox.sol";
 import {IInterchainGasPaymaster} from "../interfaces/IInterchainGasPaymaster.sol";
-import {TypeCasts} from "../libs/TypeCasts.sol";
 // ============ External Imports ============
-import {ECDSA} from "@openzeppelin/contracts/cryptography/ECDSA.sol";
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
 
 /**
@@ -20,7 +17,7 @@ contract XAppConnectionManager is Ownable {
     // ============ Public Storage ============
 
     // Outbox contract
-    Outbox public outbox;
+    IOutbox public outbox;
     // Interchain Gas Paymaster contract. The off-chain processor associated with
     // the paymaster contract must be willing to process messages dispatched from
     // the current Outbox contract, otherwise payments made to the paymaster will
@@ -122,7 +119,7 @@ contract XAppConnectionManager is Ownable {
      * @param _outbox The address of the new local Outbox contract.
      */
     function setOutbox(address _outbox) public onlyOwner {
-        outbox = Outbox(_outbox);
+        outbox = IOutbox(_outbox);
         emit NewOutbox(_outbox);
     }
 
diff --git a/solidity/core/contracts/router/XAppConnectionClient.sol b/solidity/core/contracts/router/XAppConnectionClient.sol
index 5420e6a04..bbfdcddc9 100644
--- a/solidity/core/contracts/router/XAppConnectionClient.sol
+++ b/solidity/core/contracts/router/XAppConnectionClient.sol
@@ -3,7 +3,7 @@ pragma solidity >=0.6.11;
 
 // ============ Internal Imports ============
 import {IInterchainGasPaymaster} from "../../interfaces/IInterchainGasPaymaster.sol";
-import {Outbox} from "../Outbox.sol";
+import {IOutbox} from "../../interfaces/IOutbox.sol";
 import {XAppConnectionManager} from "../XAppConnectionManager.sol";
 // ============ External Imports ============
 import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
@@ -73,7 +73,7 @@ abstract contract XAppConnectionClient is OwnableUpgradeable {
      * @notice Get the local Outbox contract from the xAppConnectionManager
      * @return The local Outbox contract
      */
-    function _outbox() internal view returns (Outbox) {
+    function _outbox() internal view returns (IOutbox) {
         return xAppConnectionManager.outbox();
     }
 
diff --git a/solidity/core/interfaces/ICommon.sol b/solidity/core/interfaces/ICommon.sol
new file mode 100644
index 000000000..f2840ccc9
--- /dev/null
+++ b/solidity/core/interfaces/ICommon.sol
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+pragma solidity >=0.6.11;
+
+interface ICommon {
+    function localDomain() external view returns (uint32);
+
+    function latestCheckpoint()
+        external
+        view
+        returns (bytes32 root, uint256 index);
+}
diff --git a/solidity/core/interfaces/IInbox.sol b/solidity/core/interfaces/IInbox.sol
new file mode 100644
index 000000000..f5ce5a172
--- /dev/null
+++ b/solidity/core/interfaces/IInbox.sol
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+pragma solidity >=0.6.11;
+
+import {ICommon} from "./ICommon.sol";
+
+interface IInbox is ICommon {
+    function checkpoint(
+        bytes32 _root,
+        uint256 _index,
+        bytes calldata _signature
+    ) external;
+
+    function remoteDomain() external returns (uint32);
+    
+    function proveAndProcess(
+        bytes calldata _message,
+        bytes32[32] calldata _proof,
+        uint256 _index
+    ) external;
+}
diff --git a/solidity/core/interfaces/IOutbox.sol b/solidity/core/interfaces/IOutbox.sol
new file mode 100644
index 000000000..4b8d2df0c
--- /dev/null
+++ b/solidity/core/interfaces/IOutbox.sol
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+pragma solidity >=0.6.11;
+
+import {ICommon} from "./ICommon.sol";
+
+interface IOutbox is ICommon {
+    function dispatch(
+        uint32 _destinationDomain,
+        bytes32 _recipientAddress,
+        bytes calldata _messageBody
+    ) external returns (uint256);
+
+    function checkpoint() external;
+}