What Is MEV in Crypto? How Bots Profit From Transaction Order

Every transaction you send on Ethereum waits in line before it lands in a block. Who sets the order of that line, and what they can earn by rearranging it, is the whole story behind MEV.

MEV stands for maximal extractable value. It is the profit available to whoever decides which transactions go into a block, which are left out and in what order they run, on top of the normal block reward and fees. Some of it keeps markets working. Some of it comes straight out of ordinary users' trades.

What MEV means in crypto

The term started life as miner extractable value. On a proof-of-work chain, miners choose the transactions for each block, so they were the ones in a position to profit from the ordering. A 2019 research paper, Flash Boys 2.0, documented trading bots on Ethereum bidding fees up against each other to get their orders placed first.

When Ethereum moved to proof of stake in September 2022, miners disappeared, but the opportunity did not. Validators now propose blocks, so the name shifted to maximal extractable value. The meaning stayed the same: value that exists only because someone gets to choose the order of transactions.

MEV is possible because blockchains are transparent before they are final. A pending transaction sits in a public waiting room, the mempool, where anyone can read it. If you can see a trade before it executes and get your own transaction placed just ahead of it or just behind it, there is often money to be made.

Who actually captures it

Validators rarely hunt for these opportunities themselves. The work is done by independent operators called searchers, who run software that scans pending transactions and on-chain data for profitable moves. Those programs are what people mean when they talk about MEV bots.

Searchers do not keep everything they find. To land in the right spot in a block, they bid for it. In crowded opportunities such as simple arbitrage between exchanges, ethereum.org notes that searchers may hand 90% or more of their revenue to the validator just to win.

Since the Merge, another layer has formed. Specialised block builders collect ordinary transactions and searchers' bundles, assemble the most valuable block they can, and bid for a validator to propose it. Most validators receive those blocks through MEV-Boost, open-source software from Flashbots, and relay data from Rated Network shows it handling around nine in ten Ethereum blocks.

The main types of MEV

Not all MEV is equally harmful. The common forms run along a spectrum, from useful housekeeping to straightforward extraction from other people's trades.

Arbitrage is the cleanest case. When a token trades at different prices on two decentralised exchanges, a bot buys on the cheaper venue and sells on the dearer one, often inside a single transaction. That closes the gap and keeps prices consistent. It is the most competitive kind of MEV and the least damaging to everyday users.

Liquidations come next. Lending protocols such as Aave let anyone repay an under-collateralised loan and claim part of the borrower's collateral as a reward. Bots race to be first. That race is painful for the borrower, but lenders depend on liquidations happening quickly, or the protocol could be left holding bad debt.

Sandwich attacks are the form that gives MEV its bad name. A bot spots a large pending swap, buys the same token just before it, lets the victim's trade push the price higher, then sells straight after. The victim pays more than they needed to, and the difference ends up with the bot.

Frontrunning covers the broader habit of copying or anticipating a profitable pending transaction and paying a higher fee to get ahead of it. Some generalised frontrunners simply replay other people's profitable transactions with their own address swapped in.

How a sandwich attack hits your swap

Say you swap 10 ETH for a token on a decentralised exchange and allow 3% slippage. That setting tells the exchange contract you will accept a price up to 3% worse than the quote you saw.

A sandwich bot reads your transaction in the mempool and works out how far your purchase will move the pool price. It places its own buy immediately before yours, nudging the price up while staying inside your tolerance. Your trade then executes at that higher price and pushes it further, and the bot's sell lands right after, near the top.

Your transaction succeeds, so nothing looks wrong. You simply receive fewer tokens than you would have without the bot in the queue. The wider your slippage tolerance and the thinner the pool, the more room there is to take.

That is why sandwiching mostly targets large swaps in shallow pools. A small trade in a deep pool usually is not worth the bot's own fees.

Why MEV matters beyond your own trade

MEV also affects the network as a whole. Before private order flow became common, bots competed by repeatedly raising their fees in public, a pattern known as priority gas auctions. According to ethereum.org, the surge in MEV during early 2021 drove gas prices to extreme levels for everyone else trying to transact.

There is a centralisation worry too. Building the most profitable blocks takes capital, data and engineering talent, and a small group of builders produces a large share of Ethereum's blocks. Large staking operations are also better placed to capture MEV than solo stakers, which adds quiet pressure toward consolidation.

On the other side of the ledger, arbitrage and liquidation bots do real work. They keep prices aligned between venues and lending markets solvent. Removing MEV entirely is not realistic. The argument is about limiting the harmful forms and sharing the rest more fairly.

What Flashbots changed

Flashbots is a research and development organisation formed in November 2020 to take MEV out of public fee wars. Its first major product was a sealed-bid auction: searchers sent bundles of transactions privately to block producers instead of broadcasting them, and losing bids never reached the chain.

Adoption was fast. By April 2021, according to Flashbots, mining pools representing more than 84% of Ethereum's hashrate were running its software. After the Merge, the same idea became MEV-Boost, which lets any validator outsource block building to a competitive market of builders.

That spread MEV income beyond the largest operators, but it introduced relays, trusted middlemen that pass blocks between builders and validators. Ethereum's upcoming Glamsterdam upgrade, expected on mainnet in late 2026, is set to move that hand-off into the protocol itself through enshrined proposer-builder separation, EIP-7732. It will not abolish MEV, but it removes the need to trust relays while it is captured.

MEV protection for ordinary users

You cannot opt out of MEV completely, but you can make yourself a far less attractive target. The simplest step is to stop sending swaps to the public mempool.

Private transaction services, often sold as MEV protection RPCs, pass your transaction directly to block builders instead. Flashbots Protect is one example. Its documentation says transactions sent through it are hidden from frontrunning and sandwich bots, can earn refunds when they create MEV, and are only included if they succeed, so you do not pay for failed transactions.

MEV Blocker works along similar lines, and some wallets and swap interfaces now route trades privately by default. Check which network endpoint your wallet actually uses before assuming you are covered.

Keep slippage as tight as the trade allows. A 0.5% setting leaves far less room for a sandwich than 5% does.

For large swaps, consider splitting the order or using an aggregator or intent-based exchange that settles trades through solvers or batch auctions rather than dropping your order straight into a pool. Deep liquidity helps for the same reason: the less your trade moves the price, the less there is to extract.

Finally, be very wary of anyone selling MEV bots as passive income. SentinelOne researchers have documented YouTube tutorials that walk viewers through deploying a supposed arbitrage bot contract which quietly sends the deposit to the scammer, with losses above $900,000 in the campaigns they tracked.

The bottom line

MEV is the price of transparent, auction-based block space. As long as anyone can see a transaction before it is final, and someone chooses the order of each block, that choice will be worth money.

For most people the practical response is narrow: send swaps through a private route, keep slippage tight, and treat ready-made MEV bots with suspicion. The protocol-level fixes are on their way, but those habits work today.