Maximal Extractable Value (MEV) is profit searchers extract by reordering, inserting, or censoring transactions within a block. In DeFi, the dominant form is the sandwich attack: a searcher front-runs a large swap (buying the asset), lets the victim's trade push price up, then back-runs (selling) for profit.

Front-run: searcher buys before victim, raising price
Victim swap: executes at worse price, paying extra slippage
Back-run: searcher sells at inflated price, capturing the spread
Defense: slippage limits, private mempools (Flashbots Protect), commit-reveal, batch auctions (CoW Swap)

MEV Type	Victim	Mitigation
Sandwich	Swapper	Tight slippage, private RPC
Liquidation	Borrower	Dutch auctions, soft liquidations
JIT Liquidity	Existing LPs	Fee tiers, minimum hold time
Backrun Arbitrage	None (beneficial)	N/A — restores price parity

Since the Merge, ~$500M+ in MEV has been extracted on Ethereum, with PBS (proposer-builder separation) routing most of it through relays like Flashbots.