diff --git a/contracts/WakuRln.sol b/contracts/WakuRln.sol
index 8ea1b92..530f14b 100644
--- a/contracts/WakuRln.sol
+++ b/contracts/WakuRln.sol
@@ -7,25 +7,46 @@ import {Ownable} from "openzeppelin-contracts/contracts/access/Ownable.sol";
 
 error NotImplemented();
 
+error InvalidIdCommitmentIndex(uint256 idCommitment, uint256 index);
+
+error MembershipStillActive(uint256 idCommitment);
+
 contract WakuRln is Ownable, RlnBase {
     uint16 public immutable contractIndex;
 
+    /// @notice The default TTL period in seconds for a membership
+    uint256 public immutable MEMBERSHIP_TTL;
+
+    /// @notice The expiry timestamp of a membership
+    /// maps from idCommitment to a timestamp
+    mapping(uint256 => uint256) public membershipExpiry;
+
     constructor(
         address _poseidonHasher,
-        uint16 _contractIndex
+        uint16 _contractIndex,
+        uint256 _ttl
     ) Ownable() RlnBase(0, 20, _poseidonHasher, address(0)) {
         contractIndex = _contractIndex;
+        MEMBERSHIP_TTL = _ttl;
+    }
+
+    function _setCommitment(uint256 idCommitment, uint256 index) internal {
+        _validateRegistration(idCommitment);
+
+        members[idCommitment] = index;
+        memberExists[idCommitment] = true;
+        membershipExpiry[idCommitment] = block.timestamp + MEMBERSHIP_TTL;
+
+        emit MemberRegistered(idCommitment, index);
     }
 
     /// Registers a member
     /// @param idCommitment The idCommitment of the member
     function _register(uint256 idCommitment) internal {
-        _validateRegistration(idCommitment);
+        if (idCommitmentIndex >= SET_SIZE) revert FullTree();
 
-        members[idCommitment] = idCommitmentIndex;
-        memberExists[idCommitment] = true;
+        _setCommitment(idCommitment, idCommitmentIndex);
 
-        emit MemberRegistered(idCommitment, idCommitmentIndex);
         idCommitmentIndex += 1;
     }
 
@@ -39,6 +60,27 @@ contract WakuRln is Ownable, RlnBase {
         }
     }
 
+    /// Register a member at specific index
+    /// @param idCommitment The idCommitment of the member
+    /// @param index The index in which to register the member
+    function registerAtIndex(uint256 idCommitment, uint256 index) external onlyOwner {
+        _validateIndexExpiration(idCommitment, index);
+        _setCommitment(idCommitment, index);
+    }
+
+    /// Renew membership credentials
+    function renew(uint256 idCommitment /*, uint256 periods*/) external onlyOwner {
+        // TODO: should this function be payable, and also, accept renewals for more than 1 period?
+        // if (msg.value != periods * MEMBERSHIP_DEPOSIT) revert InvalidTransactionValue()
+
+        // TODO: should we allow renewals using a grace period?
+
+        uint256 expiry = membershipExpiry[idCommitment];
+        if (expiry == 0) revert InvalidIdCommitment(idCommitment);
+
+        membershipExpiry[idCommitment] += MEMBERSHIP_TTL /* * periods */;
+    }
+
     function register(uint256 idCommitment) external payable override {
         revert NotImplemented();
     }
@@ -57,7 +99,14 @@ contract WakuRln is Ownable, RlnBase {
         if (!isValidCommitment(idCommitment))
             revert InvalidIdCommitment(idCommitment);
         if (memberExists[idCommitment] == true) revert DuplicateIdCommitment();
-        if (idCommitmentIndex >= SET_SIZE) revert FullTree();
+    }
+
+    function _validateIndexExpiration(
+        uint256 idCommitment,
+        uint256 index
+    ) internal view {
+        if (index >= SET_SIZE) revert InvalidIdCommitmentIndex(idCommitment, index);
+        if (membershipExpiry[idCommitment] > block.timestamp) revert MembershipStillActive(idCommitment);
     }
 
     function _validateSlash(