In the previous chapter, we briefly introduced the smart contract concept as self-executing programs that define a set of clauses stored on the blockchain. Although bitcoin was the first blockchain to propose a scripting language (limited for security reasons) to control funds, the smart contract concept is usually associated with the Ethereum blockchain, which provides Turing-complete smart contract languages. However, to redress the balance, many projects were initiated, such as Rootstock, Counterparty, and Ivy, to enable building and running advanced smart contracts on bitcoin. Welcome to bitcoin 2.0.