This book is a revised and updated version of the author's 1997 Cornell University dissertation, which investigated the acquisition of Spanish past-tense morphology by classroom learners whose L1 was English. Based on three main studies, the author argues against the Aspect Hypothesis (e.g., Bardovi-Harlig, 2000), the dominant view in SLA research, which predicts strong associations between lexical aspect and tense-aspect morphology at the early stages of acquisition.