Twenty years ago, a state-of-the-art review in SSLA marked the coming of age of the study of temporality in second language acquisition. This was followed by three monographs on tense and aspect the next year. This article presents a state-of-the-scholarship review of the last 20 years of research addressing the aspect hypothesis (AH) (Andersen, 1991, 2002; Andersen & Shirai, 1994, 1996), the most tested hypothesis in L2 temporality research. The first section of the article gives an overview of the AH and examines its central tenets, and then explores the results of empirical studies that test the hypothesis. The second section considers studies that have investigated four crucial variables in the acquisition of temporality and the testing of the AH. The third section discusses theoretically motivated areas of future research within the framework of the hypothesis.