This book provides a state-of-the-art review of the acquisition of lexical and grammatical aspect, in both first and second language acquisition. More specifically, it presents a comprehensive analysis of how child and adult speakers learn to mark aspect, an important subsystem of language that marks the temporal contour of events by means of inherent lexical meanings and/or grammatical morphology (in contrast to tense which marks the temporal location of events with respect to past, present, and future). The studies presented are based on the authors' research on English, Chinese, and Japanese, and they address the issue of the acquisition of aspect from a number of different perspectives, among them crosslinguistic, developmental, and computational. Detailed empirical results are integrated with theoretical analyses and syntheses, along dimensions such as innateness versus input, prototypes versus cryptotypes, rules versus connections. Linguistically, the authors' approach to aspectual phenomena relies on the interaction between lexical aspect (e.g. state, activity, accomplishment, and achievement) and grammatical aspect (e.g. perfective, imperfective, and progressive). Developmentally, their approach to acquisition phenomena relies on connectionist distributional learning that gives rise to categories of protototypes and cryptotypes. Readers from linguistics, psychology, language acquisition, language education, and cognitive science should all find this book a relevant and important text for their research and teaching.