diff --git a/CHANGELOG.md b/CHANGELOG.md
index 868a253..059b675 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,7 +8,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ## [unreleased]
 
 ### Changed
- -  Using new [Signature Validator](https://github.com/DexyProject/SignatureValidator) package.
+ - Solidity version 0.4.23
+ - Using new [Signature Validator](https://github.com/DexyProject/SignatureValidator) package.
 
 ## [2.1.0] - 2018-04-26
  
diff --git a/contracts/Exchange.sol b/contracts/Exchange.sol
index 707bf5c..80f1610 100644
--- a/contracts/Exchange.sol
+++ b/contracts/Exchange.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 import "./ExchangeInterface.sol";
 import "./Libraries/OrderLibrary.sol";
@@ -17,7 +17,7 @@ contract Exchange is Ownable, ExchangeInterface {
 
     ExchangeLibrary.Exchange public exchange;
 
-    function Exchange(uint _takerFee, address _feeAccount, VaultInterface _vault) public {
+    constructor(uint _takerFee, address _feeAccount, VaultInterface _vault) public {
         require(address(_vault) != 0x0);
         setFees(_takerFee);
         setFeeAccount(_feeAccount);
diff --git a/contracts/ExchangeInterface.sol b/contracts/ExchangeInterface.sol
index 976dff8..1347316 100644
--- a/contracts/ExchangeInterface.sol
+++ b/contracts/ExchangeInterface.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 import "./Vault/VaultInterface.sol";
 
diff --git a/contracts/HookSubscriber.sol b/contracts/HookSubscriber.sol
index 53a9444..6a3bf91 100644
--- a/contracts/HookSubscriber.sol
+++ b/contracts/HookSubscriber.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 interface HookSubscriber {
 
diff --git a/contracts/Libraries/ExchangeLibrary.sol b/contracts/Libraries/ExchangeLibrary.sol
index ac60775..a24c4db 100644
--- a/contracts/Libraries/ExchangeLibrary.sol
+++ b/contracts/Libraries/ExchangeLibrary.sol
@@ -2,9 +2,9 @@ pragma solidity ^0.4.23;
 
 import "./OrderLibrary.sol";
 import "./SafeMath.sol";
-import "@dexyproject/signature-validator/contracts/SignatureValidator.sol";
 import "./../Vault/VaultInterface.sol";
 import "./../HookSubscriber.sol";
+import "@dexyproject/signature-validator/contracts/SignatureValidator.sol";
 
 library ExchangeLibrary {
 
diff --git a/contracts/Libraries/OrderLibrary.sol b/contracts/Libraries/OrderLibrary.sol
index 4650a9c..9b583f5 100644
--- a/contracts/Libraries/OrderLibrary.sol
+++ b/contracts/Libraries/OrderLibrary.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 library OrderLibrary {
 
diff --git a/contracts/Libraries/SafeMath.sol b/contracts/Libraries/SafeMath.sol
index 9f46269..8dcd439 100644
--- a/contracts/Libraries/SafeMath.sol
+++ b/contracts/Libraries/SafeMath.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 library SafeMath {
 
diff --git a/contracts/Ownership/Ownable.sol b/contracts/Ownership/Ownable.sol
index 38df788..2bfadba 100644
--- a/contracts/Ownership/Ownable.sol
+++ b/contracts/Ownership/Ownable.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 contract Ownable {
 
@@ -9,7 +9,7 @@ contract Ownable {
         _;
     }
 
-    function Ownable() public {
+    constructor() public {
         owner = msg.sender;
     }
 
diff --git a/contracts/Tokens/ERC20.sol b/contracts/Tokens/ERC20.sol
index 4d31858..26edf70 100644
--- a/contracts/Tokens/ERC20.sol
+++ b/contracts/Tokens/ERC20.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 interface ERC20 {
 
diff --git a/contracts/Tokens/ERC777.sol b/contracts/Tokens/ERC777.sol
index d72e585..2df3c7f 100644
--- a/contracts/Tokens/ERC777.sol
+++ b/contracts/Tokens/ERC777.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 interface ERC777 {
     function name() public constant returns (string);
diff --git a/contracts/Vault/Vault.sol b/contracts/Vault/Vault.sol
index 39047c3..bdb6e10 100644
--- a/contracts/Vault/Vault.sol
+++ b/contracts/Vault/Vault.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 import "./VaultInterface.sol";
 import "../Interfaces/ERC820.sol";
@@ -11,6 +11,11 @@ contract Vault is Ownable, VaultInterface {
 
     using SafeMath for *;
 
+    string constant public NOT_SPENDER_ERROR = "NOT_SPENDER_ERROR";
+    string constant public NOT_APPROVED_ERROR = "NOT_APPROVED_ERROR";
+    string constant public ERC20_TRANSFER_FAILED = "ERC20_TRANSFER_FAILED";
+    string constant public INSUFFICIENT_BALANCE_ERROR = "INSUFFICIENT_BALANCE_ERROR";
+
     address constant public ETH = 0x0;
 
     mapping (address => bool) public isERC777;
@@ -24,16 +29,16 @@ contract Vault is Ownable, VaultInterface {
     address private latest;
 
     modifier onlySpender {
-        require(spenders[msg.sender]);
+        require(spenders[msg.sender], NOT_SPENDER_ERROR);
         _;
     }
 
     modifier onlyApproved(address user) {
-        require(approved[user][msg.sender]);
+        require(approved[user][msg.sender], NOT_APPROVED_ERROR);
         _;
     }
 
-    function Vault(ERC820 registry) public {
+    constructor(ERC820 registry) public {
         // required by ERC777 standard.
         registry.setInterfaceImplementer(address(this), keccak256("ERC777TokensRecipient"), address(this));
     }
@@ -48,7 +53,7 @@ contract Vault is Ownable, VaultInterface {
         if (token == ETH) {
             value = msg.value;
         } else {
-            require(ERC20(token).transferFrom(msg.sender, address(this), value));
+            require(ERC20(token).transferFrom(msg.sender, address(this), value), ERC20_TRANSFER_FAILED);
         }
 
         depositFor(msg.sender, token, value);
@@ -58,7 +63,7 @@ contract Vault is Ownable, VaultInterface {
     /// @param token Address of the token to withdraw.
     /// @param amount Amount of tokens to withdraw.
     function withdraw(address token, uint amount) external {
-        require(balanceOf(token, msg.sender) >= amount);
+        require(balanceOf(token, msg.sender) >= amount, INSUFFICIENT_BALANCE_ERROR);
 
         balances[token][msg.sender] = balances[token][msg.sender].sub(amount);
         accounted[token] = accounted[token].sub(amount);
@@ -204,6 +209,6 @@ contract Vault is Ownable, VaultInterface {
             return;
         }
 
-        require(ERC20(token).transfer(user, amount));
+        require(ERC20(token).transfer(user, amount), ERC20_TRANSFER_FAILED);
     }
 }
diff --git a/contracts/Vault/VaultInterface.sol b/contracts/Vault/VaultInterface.sol
index 529f1c1..dcb1777 100644
--- a/contracts/Vault/VaultInterface.sol
+++ b/contracts/Vault/VaultInterface.sol
@@ -1,4 +1,4 @@
-pragma solidity ^0.4.21;
+pragma solidity ^0.4.23;
 
 interface VaultInterface {