ERC-20 is a technical standard used for smart contracts on the Ethereum blockchain for implementing tokens. The majority of tokens issued on the Ethereum blockchain are ERC-20 compliant. As of April 23, 2019, a total of 182,771 ERC-20 compatible tokens are found on Ethereum main network. ERC-20 defines a common list of rules for Ethereum tokens to follow within the larger Ethereum ecosystem, allowing developers to accurately predict interaction between tokens. These rules include how the tokens are transferred between addresses and how data within each token is accessed.