This article presents the results of a study investigating the acquisition of the simple past tense, identifies areas of difficulty, and presents an acquisitionally based approach to instruction for the problematic areas. The study, a cross-sectional investigation of 182 adult learners of English as a second language at six levels of proficiency, showed that the acquisition of the past tense in English is not a unitary phenomenon, but that it proceeds in stages. These stages are determined by the meaning of verbs as they relate to the expression of action and time, what we will term lexical aspect. These findings show that the acquisition of tense by classroom language learners follows the same sequences of development (with instruction) that have been observed in the acquisition of adult learners and in children without instruction. In early stages, learners often do not use the past tense where it is preferred by native speakers, indicating an undergeneralization of the meaning of the past in the learner grammar. We present an approach to instruction aimed at increasing the use of the past to balance contextualized examples through the use of authentic text and focused noticing exercises to encourage the learners toward a more targetlike association of form and meaning.