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What is MEV?
MEV (maximal extractable value) is the profit that can be made by choosing which transactions go into a block and in what order, beyond normal fees and rewards. A common form is the sandwich: someone sees a large pending trade, buys just before it and sells just after.
What it means for you
If your swap is sandwiched, you get a worse price than you expected, up to the slippage limit you set. A wide slippage tolerance on a public pending trade gives more room for this.
How it works
Block producers choose which pending transactions go in a block and in what order. Searchers run programs that watch the mempool for profitable orderings: arbitrage between exchanges, being first to liquidate a loan, or a sandwich, where a buy goes just ahead of a large swap and a sell just after it. They compete by bidding gas fees of up to all of their expected profit. Slippage is the price gap a trader ends up with; a sandwich turns part of that allowed slippage into the searcher's profit. Proposer-builder separation hands block building to competing builders.
An example
Say you submit a swap with 2% slippage tolerance. A searcher buys just before you, pushing the price up 1.8%; your swap still fills, at that worse price, because it is inside your limit. The searcher then sells into the higher price your trade created and keeps the difference.
Source: ethereum.org: Maximal extractable value · checked 4 October 2026
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