Comparing universal against language-specific factors, this chapter examines the roles of lexical aspect, morphological regularity, and transfer in the developmental emergence of past and progressive morphology among four adult learners of English from Italian and Punjabi L1 backgrounds. The learner production data were obtained from the European Science Foundation SLA corpus (Perdue 1993). In contrast to qualitative findings by Klein (1995), quantitative results reveal that lexical aspect correlates with the distribution of tense-aspect morphology, supporting the core predictions of the Aspect Hypothesis (Andersen and Shirai 1994) that learners predominantly use past/perfective markers with telic predicates, and progressive morphology with activity verbs. A paucity of production data makes it difficult to pinpoint effects of morphological regularity and transfer.