Yield lies at the core of decentralized finance. More than any other measure, it is the lens through which investors, protocols, and users assess the attractiveness of opportunities. Yet, behind what may appear as a simple percentage figure, the mechanisms that generate yield differ widely. Some are rooted in genuine economic activity, such as trading fees or borrowing costs, while others are driven primarily by incentives or token distributions that may prove temporary. The purpose of this chapter is to establish a foundation. It defines what yield means in DeFi, why it exists, and what its main sources are. By examining lending, liquidity provision, staking, and fixed-rate instruments, we highlight the diverse structures through which yield is produced and the implications these have for portfolio management. At this stage, the goal is not to pass judgment on the sustainability or desirability of specific forms of yield, that task belongs to later chapters. Instead, the aim is to provide a clear mapping of the fundamental mechanisms, ensuring that subsequent analysis can build on a consistent understanding of where yield comes from in decentralized finance.