Library · Crypto, word by word · Tokens and supply
What is ERC-721?
ERC-721 is the Ethereum standard for non-fungible tokens, where each token is one of a kind. Every token has its own ID number inside its contract, and the standard defines how to look up its owner, transfer it, and point to a description of what it represents.
What it means for you
The token usually points to its image or file through a link (the tokenURI) rather than holding it; if that link goes offline, the picture can vanish while you still own the token. The setApprovalForAll function hands one address control of every token you hold from that contract, so granting it to the wrong address can cost you all of them.
A common mistake: “The NFT itself contains the image.”
In fact: Usually the token holds an ID and a tokenURI pointing to metadata stored elsewhere. The standard allows that URI to change, so what the token displays can change while the token stays yours.
How it works
Each NFT is identified by the pair of its contract address and its tokenId, which together are globally unique. An ownerOf function returns the holder, and the standard's checked transfer confirms that a receiving contract implements onERC721Received, reverting otherwise. Approvals come in two forms: approve sets one address for one token, while setApprovalForAll gives an operator control of all your tokens in that contract. An optional tokenURI points to JSON metadata with a name, description and image, and the standard allows that URI to change over time.
Source: EIP-721: Non-Fungible Token Standard · checked 4 October 2026
Often confused with
Related words
Educational content, not financial advice. Written by hand and checked against the source named above. Something wrong? Tell us and we reply within two business days.