Mempool
The mempool (memory pool) is the set of pending transactions that have been broadcast to the network but not yet included in a block. In Ethereum, the mempool is a public, transparent data structure — any node operator can observe all pending transactions, their gas prices, and their calldata. This transparency is what enables MEV: searchers scan the mempool for profitable trades to front-run, sandwich, or back-run. Private mempools (also called dark pools or MEV-protection RPCs) route transactions directly to builders or relays, hiding them from the public mempool until inclusion. The shift toward private order flow — some estimates suggest over 90% of MEV-relevant transaction value now bypasses the public mempool — has fundamentally changed Ethereum's transaction supply chain. Mempool analysis is also a powerful on-chain intelligence tool: observing pending transactions reveals trader behavior, bot activity, and potential market-moving events seconds to minutes before they settle on-chain.