This study develops and tests an integrated model that explains information systems usage at adoption and post-adoption stages. Integrating the theory of planned behavior with the IS continuance model, this research provides insights into the complementary nature of the two theories in explaining IS usage over time. In addition, the integrated model takes into consideration the “habit” construct in order to better explain the automatic nature of IS continuance. The integrated model is tested in a longitudinal setting. Results present strong support for the existing theoretical links of the theory of planned behavior and IS continuance model, as well as for those newly hypothesized in this study. Initial usage indicates significant affect IS continuance usage, and habit posits a significant moderating impact on the relationship between IS continuance intention and IS continuance usage. The dynamic interrelationship between acceptance and continuance decision and the variability of determinants of IS usage across different adoption stages are demonstrated. The implications are noteworthy for both researchers and practitioners.