A mempool, short for “memory pool,” is the collection of unconfirmed transactions a blockchain node has received and is temporarily storing. You can think of it as a waiting room between the moment a user broadcasts a transaction and the moment that transaction is confirmed by being included in a block.
How the mempool works
When you send a Bitcoin or Ethereum transaction, it is propagated across the peer-to-peer network. Each node independently checks basic validity rules, such as correct signatures and sufficient funds or nonce ordering, then places the transaction into its mempool if it passes. Miners or validators typically select transactions from their mempool to build the next block, prioritizing those that are most attractive under the chain’s fee mechanism.
Importantly, there is no single, global mempool. Every node maintains its own view based on which transactions it has seen, which it considers valid, and how it applies policies like minimum fees, size limits, or expiry rules. This is why a transaction might appear as “pending” in one explorer yet be missing from another node’s mempool.
Fees, congestion, and real-world behavior
Mempool dynamics help explain why confirmations can be fast sometimes and slower at others. During periods of heavy demand, mempools can become crowded, and users who attach higher fees are generally picked first for inclusion in blocks. If a fee is too low relative to current conditions, a transaction may remain pending for a long time or even be dropped from some nodes’ mempools.
The mempool is also where transaction ordering competition plays out. On some networks, sophisticated actors monitor mempools to react to pending trades or liquidations, which is closely related to miner or maximal extractable value (MEV).
Why it matters
Understanding the mempool helps users interpret “pending” status, choose appropriate fees, and grasp how network congestion and transaction ordering affect security and user experience across blockchains.