The 2017 CryptoKitties digital-collectibles game was hosted on the Ethereum blockchain.
CryptoKitties was launched by Canadian studio AxiomZen in November 2017. Players could buy, sell, breed, and collect unique virtual cats represented by blockchain tokens. The game used Ethereum smart contracts to record ownership and manage its breeding rules.
CryptoKitties became famous because its popularity contributed to severe congestion on Ethereum in December 2017. Transactions became slower and more expensive as users competed to interact with the game. The episode demonstrated both the potential and the scaling limits of a public blockchain being used for applications beyond payments.
CryptoKitties is often described as an early mainstream example of non-fungible tokens, although the term NFT became much more widely used later. Its assets were not interchangeable like ordinary currency tokens: each cat had distinct traits and a separate ownership record.