This book presents an extended analysis of the development of L2 Spanish past tense morphology among L1 English-speaking learners. The study addresses three major questions: (1) what is the developmental pattern of acquisition of past tense verbal morphology among tutored learners? (2) what are the relevant factors that may account for the particular distribution of morphological endings (especially at the beginning stages)?, and (3) how does instruction affect the movement from one stage to the next? The analysis provides a reassessment of the general claim of Andersen's lexical aspect hypothesis and proposes minor changes that may render the hypothesis more appropriate for, especially, L2 classroom learning. The study includes an overview of theoretical positions on the notion of lexical versus grammatical aspect, and a comparison of the findings from previous empirical studies on the development of past tense verbal morphology among both classroom and naturalistic learners.