Over twenty years have passed since Nelson and Winter put the concept of routines firmly at the center of the analysis of organizational and economic change. A growing number of researchers have followed their example since. However, researchers have not always had the same idea of what routines are and what effect they have on organizations. Over time, this has left the literature on routines riddled with ambiguities. For researchers who want to apply the concept of routines in their research, it is not easy to get an overview of the current thinking about routines and their effects. This article offers a systematic review of the literature that has contributed to the theoretical development of the concept of routines, and of the empirical literature that has applied the concept of routines.