ERC-721

Overview

ERC-721 is the Ethereum interface standard for non-fungible tokens, defining common functions and events for identifying unique token IDs, checking ownership, granting approvals, and transferring NFTs. NFT ownership identifies control of a token under a specific contract or protocol; it does not automatically transfer copyright, physical title, service performance, or other off-chain rights.

The standard defines an interface, not a guarantee of metadata quality, scarcity, royalties, security, or legal rights. Individual contracts may add minting, pausing, upgradeability, enumeration, or custom transfer logic. It should be read alongside ERC-1155, NFT Collection, NFT Minting. These concepts describe adjacent but different layers of the asset, so substituting one for another can hide the governing network, holder claim, authority, supply measure, or operational action.

Operationally, integrators verify the chain and contract, ERC-165 support, ownerOf and transfer behavior, approvals, safe receiver checks, metadata URI handling, events, proxy implementation, and collection-specific extensions. A production system should preserve the network, contract or asset identifier, units and precision, governing rule version, responsible authority, effective timestamp, and transaction or external record used to support the state shown to a user. Changes should be observable, reconciled, and tested across deposits, transfers, withdrawals, upgrades, and exceptional cases.

Smart-contract defects, misleading metadata, unauthorized minting, marketplace impersonation, illiquid markets, custody mistakes, and uncertainty over off-chain rights can reduce or eliminate practical value. Teams should test failed transactions, unavailable indexers or external services, compromised keys, stale metadata or prices, contract and protocol upgrades, chain reorganizations, role changes, and inconsistent records between blockchain, market, custody, legal, and accounting systems.

For custody or marketplace support, verify chain, canonical contract or collection, token ID, ownership, approvals, metadata source, transfer behavior, royalties, and off-chain terms. Monitoring should cover privileged-role events, supply or ownership changes, contract migrations, parameter updates, redemption or transfer exceptions, and evidence that the represented rights remain enforceable. This makes ERC-721 an auditable operational concept rather than a label accepted only from a wallet, marketplace, or issuer interface.