ERC-721

ERC-721 is the Ethereum Improvement Proposal that gives every non-fungible token a permanent, verifiable identity on the Ethereum blockchain. Drafted as EIP-721 by developers William Entriken, Dieter Shirley, Jacob Evans, and Nastassia Sachs and formally finalized in June 2018, the standard grew directly out of CryptoKitties, the breeding-and-trading game that first proved the market wanted a shared way to represent one-of-a-kind digital assets.

Technically, an ERC-721 smart contract assigns every token a unique tokenId paired with its contract address, so no two tokens are interchangeable even within the same collection, unlike fungible standards where each unit is identical. The interface defines a common set of functions, including ownerOf, transferFrom, and approve, so wallets, marketplaces, and applications can read and move any compliant token without writing custom code for each project. Most ERC-721 contracts also implement ERC-165, a compatibility check that stops tokens from becoming permanently stuck in contracts unable to handle them.

The standard underpins most major NFT use cases: digital art and collectibles, profile-picture projects such as Bored Ape Yacht Club, in-game items and virtual land, event tickets, and domain names. Each token typically carries metadata, often a link to an image plus a set of attributes, that marketplaces display alongside cryptographic proof of ownership and transfer history.

Its main limitation is efficiency: transferring or minting many tokens requires a separate transaction and gas fee for each one, since one contract manages exactly one asset type. The newer ERC-1155 standard addresses this with batch transfers that can cut gas costs sharply for bulk operations, yet ERC-721 remains the dominant choice for flagship, one-of-a-kind collections thanks to its simplicity and the depth of wallet and marketplace support built around it.