MEV: maximum extractable value

The invisible tax in your transactions

MEV stands for Maximum Extractable Value. It used to be called Miner Extractable Value before Ethereum switched to validators in 2022. The idea is simple and a little uncomfortable. When a validator builds the next block, they get to choose which transactions to include and what order to put them in. That ordering power is worth money. And there's an entire industry of bots competing for it.

Here's how it works at the basic level. The mempool — the public waiting room you learned about in lesson one — is visible to everyone. Bots constantly scan it for transactions they can profit from. They see a big DeFi swap about to happen? They might insert their own buy before it and sell after, extracting profit from the price movement the victim's trade was about to cause. They see a DeFi position about to fall below its collateral ratio? They race to be the first to liquidate it. They see an arbitrage opportunity between two exchanges? They race to grab it.

MEV isn't a hack or an exploit in the legal sense. The system is technically working as designed. Bots compete by offering validators a cut of the profit they extract — sometimes a huge cut. Validators include the profitable bundles. Everyone in that chain makes money. The party that pays for it, often invisibly, is the retail trader whose trade got front-run or sandwiched.

Three forms of MEV worth knowing by name. First, arbitrage — bots correcting price differences between exchanges. Mostly harmless to retail; it's bot-on-bot competition. Second, liquidations — bots racing to liquidate undercollateralized DeFi positions. Affects DeFi borrowers, not casual traders. Third, sandwich attacks — the one most retail traders quietly fund. A bot sees your pending swap, places a buy in front of it, lets your transaction move the price, then sells immediately after. You absorb the slippage, the bot keeps the difference. That's the next lesson.

Recap: MEV = profit from controlling transaction order in a block. Bots watch the public mempool, identify profitable patterns, pay validators to include their bundles. Arbitrage and liquidations are mostly bot-on-bot. Sandwich attacks are the one that costs retail directly.