This chapter reviews and synthesizes the current understanding of the shocks that drive economic fluctuations. It begins by illustrating the problem of identifying macroeconomic shocks and then surveys recent innovations in shock identification. The chapter reviews three main types of shocks: monetary, fiscal, and technology shocks. Each section presents new estimates that compare and synthesize key parts of the literature. The chapter concludes by analyzing the extent to which leading shock candidates can explain fluctuations in output and hours, suggesting that we are much closer to understanding the shocks that drive economic fluctuations than we were twenty years ago. The chapter discusses various methods for identifying shocks and estimating impulse responses, including Cholesky decompositions, narrative methods, high-frequency identification, external instruments/proxy SVARs, long-run restrictions, sign restrictions, factor-augmented VARs, and estimated DSGE models. It also addresses the problems of foresight and trends in identifying shocks and briefly touches on the importance of nonlinearities.This chapter reviews and synthesizes the current understanding of the shocks that drive economic fluctuations. It begins by illustrating the problem of identifying macroeconomic shocks and then surveys recent innovations in shock identification. The chapter reviews three main types of shocks: monetary, fiscal, and technology shocks. Each section presents new estimates that compare and synthesize key parts of the literature. The chapter concludes by analyzing the extent to which leading shock candidates can explain fluctuations in output and hours, suggesting that we are much closer to understanding the shocks that drive economic fluctuations than we were twenty years ago. The chapter discusses various methods for identifying shocks and estimating impulse responses, including Cholesky decompositions, narrative methods, high-frequency identification, external instruments/proxy SVARs, long-run restrictions, sign restrictions, factor-augmented VARs, and estimated DSGE models. It also addresses the problems of foresight and trends in identifying shocks and briefly touches on the importance of nonlinearities.