The acquisition of the aspect is a central area in Second Language Acquisition research, the subject of hundreds of papers and dozens of edited volumes, monographs and special issues. This introduction provides the reader not only with a concise and plain presentation of the main hypotheses advanced in the past, but also with an overview of contemporary research. Stefano Rastelli shows how comparison of behavioural (production-comprehension), processing and statistical data is improving -- and partially changing -- our understanding of how learners acquire the aspectual distinctions of the target-language.