This chapter offers an overview of diachronic accounts of aspectual change, focusing on their treatment of the relationship between Aktionsart and perfectivity, often referred to as (grammatical) aspect. Such studies, relatively rare, are important in several ways: from the point of view of the diachronic study of language they offer explanations of broader changes in the verbal systems of individual languages, and they enhance our understanding of the mechanisms of language change in general. From the point of view of the study of aspect, they can shed light on the nature of the two categories in question, on their common ground and the differences between them and on their relationship with the other time-related category, tense. The chapter will focus on Greek, one of the Indo-European languages that are widely thought of as being of particular interest for the reconstruction of the Proto-Indo-European verbal system, but it will touch upon the significance of the diachronic study of aspect for the theoretical analysis of aspect and tense.