> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Front-Running

> The practice of placing a transaction before a known pending transaction to profit from or alter its expected execution.

# Front-Running

The practice of placing a transaction before a known pending transaction to profit from or alter its expected execution.

## Definition

The practice of placing a transaction before a known pending transaction to profit from or alter its expected execution.

## Why it matters

MEV concepts explain how transaction ordering can create economic opportunities and risks.

## How it works

Bots constantly scan the pending transaction pool (mempool) for large trades. When a large trade is detected, the bot sends a transaction to buy the same asset, paying a higher gas fee to be included in the same or previous block. Once the user's trade pushes the price up, the bot sells the asset for a profit.

## Real-world example

Uniswap users often experience 'sandwich attacks,' where a bot buys before a user's trade and sells immediately after, trapping the user in a profit-stealing loop.

## Advantages

* Provides market efficiency for arbitrageurs
* Incentivizes faster transaction processing
* Highlights the need for better trade protections

## Limitations

* Harms retail user experience
* Increases transaction costs through gas wars
* Can lead to high slippage for users

## Common misconceptions

* Front-running is not always technically illegal in decentralized finance, but it is considered an extraction of user value.
* Users believe paying higher gas fees always prevents front-running, but bots simply adjust their fees to remain ahead.

## Related knowledge

* [Block Builder](/generated/v2/glossary/block-builder) — term
* [Mempool](/generated/v2/glossary/mempool) — term
* [Searcher](/generated/v2/glossary/searcher) — term
* [Transaction Ordering](/generated/v2/glossary/transaction-ordering) — term

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**Canonical knowledge ID:** `glossary:front-running`