diff --git a/BasicBlockChain.Core/BlockChain.cs b/BasicBlockChain.Core/BlockChain.cs
index 7406ebd..e6a6a49 100644
--- a/BasicBlockChain.Core/BlockChain.cs
+++ b/BasicBlockChain.Core/BlockChain.cs
@@ -11,12 +11,66 @@ namespace BasicBlockChain.Core
         public int Difficulty { get; set; } = 2;
         public int Reward { get; set; } = 1_000_000;
 
+        private List<string> _users = null;
+
+        public List<string> Users
+        {
+            get
+            {
+                if (_users == null)
+                {
+                    InitUsers();
+                }
+                return _users;
+            }
+        }
+
+        private void InitUsers()
+        {
+            _users = new List<string>();
+            foreach (Block block in Chain)
+            {
+                foreach (Transaction transaction in block.Transactions)
+                {
+                    AddUser(transaction.Sender, false);
+                    AddUser(transaction.Receiver, false);
+                }
+            }
+        }
+
+        private void AddUser(string user, bool init = true)
+        {
+            if (string.IsNullOrEmpty(user)) { return; }
+            if (_users == null)
+            {
+                if (init)
+                {
+                    InitUsers();
+                }
+                else
+                {
+                    return;
+                }
+            }
+            if (_users.Contains(user)) { return; }
+            _users.Add(user);
+        }
+
+        private void AddUsersOfBlock(Block block)
+        {
+            foreach (Transaction transaction in block.Transactions)
+            {
+                AddUser(transaction.Sender);
+                AddUser(transaction.Receiver);
+            }
+        }
+
         public BlockChain(DateTime? genesisDate = null, int difficulty = 2, int reward = 1_000_000)
         {
             Block genesisBlock = new Block(genesisDate ?? DateTime.UtcNow, null, null);
             genesisBlock.Mine(difficulty);
             Difficulty = difficulty;
-            Reward = 1_000_000;
+            Reward = reward;
             Chain.Add(genesisBlock);
         }
 
@@ -25,6 +79,7 @@ namespace BasicBlockChain.Core
             Block lastBlock = Chain.Last();
             Block newBlock = new Block(date, lastBlock, transactions);
             newBlock.Mine(Difficulty);
+            AddUsersOfBlock(newBlock);
             Chain.Add(newBlock);
         }
 
@@ -39,6 +94,7 @@ namespace BasicBlockChain.Core
             Block newBlock = new Block(date, lastBlock, PendingTransactions);
             newBlock.Transactions.Add(new Transaction(null, miner, Reward, date));
             newBlock.Mine(Difficulty);
+            AddUsersOfBlock(newBlock);
             Chain.Add(newBlock);
             PendingTransactions.Clear();
         }