Mempool

The mempool (memory pool) is the waiting room for blockchain transactions: the set of valid, signed transactions that have been broadcast to the network but not yet included in a block. When you send a transaction, your wallet transmits it to nodes, each of which checks it and, if valid, holds it in its own mempool and relays it onward — until a miner or validator selects it for the next block.

There is no single global mempool; every node maintains its own view, which mostly overlaps but can differ based on relay policies, minimum fee settings, and network timing. Block producers generally pick the transactions paying the highest fees per unit of space, so the mempool functions as an open fee auction. For example, during a surge of Bitcoin activity the mempool can swell with hundreds of thousands of pending transactions, and a payment sent with a low fee may wait hours or days — or be dropped entirely — while higher bidders jump the queue.

A common misconception is that a transaction "in the mempool" is partially confirmed; it has zero confirmations, is not part of the chain, and can still be replaced or double-spent, which is why merchants should never treat unconfirmed transactions as final. The mempool is also where front-running happens on smart-contract chains: because pending transactions are publicly visible, bots watch the pool and pay higher fees to get their own transactions ordered ahead of profitable targets — a dynamic known as MEV.