In this chapter, you will learn how Ethereum expanded the vision of blockchains beyond cryptocurrencies by introducing smart contracts and decentralized applications. We begin by exploring Ethereum's motivation and architecture, especially its support for storing and executing Turing-complete code on-chain. You will understand the role of the Ethereum Virtual Machine, the structure and function of smart contracts, and the transaction types specific to Ethereum, including contract creation and internal transactions. We examine how Ethereum handles gas costs, gas limits, and storage operations, with a focus on optimizing efficiency and preventing abuse. You will also learn how Ethereum transactions trigger computation and how message calls allow contracts to interact. Technical details such as OpCodes, memory types (stack, memory, storage), calldata, and the Application Binary Interface are covered to help you understand contract execution. We discuss Ethereum’s consensus mechanisms, including the transition from Proof of Work to Proof of Stake, and the use of Merkle Patricia Tries to store state and transaction data. Finally, the chapter introduces concepts such as upgradable contracts, node types, event logging, and the Web3 ecosystem of decentralized applications.