> For the complete documentation index, see [llms.txt](https://dev.bondprotocol.finance/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dev.bondprotocol.finance/smart-contracts/bond-system/callback.md).

# Callback

The Callback contract is an optional feature of the Bond system. 

Callbacks allow issuers (market creators) to apply custom logic on receipt and payout of tokens. The Callback must be created prior to market creation and the address passed in as an argument. The Callback depends on the Aggregator contract for the Auctioneer that the market is created with to get market data.

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