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.