This article presents a social psychological model of prospective memory and habit development. The model is based on relevant research literature, and its dynamics were investigated by computer simulations. Time-series data from a behavior-change campaign in Cuba were used for calibration and validation of the model. The model scored well in several system-analytical tests, including the replication of the data and the forecast of later developments based on earlier data. Additionally, the calibrated parameter values indicate that the accessibilities of intentions decay at the same rate as retrospective memories. However, the accessibilities may stay high due to a reminder, the effectiveness of which depends on a person's commitment to performing the behavior. Furthermore, the effect of the reminder decays over time. This decay is much slower than the development of habits, which, after about a month, were nearly fully developed if the person had executed the behavior sufficiently often. Finally, over time, habits were shown to replace the reminding effect of the external memory aid. This article points to a new understanding of the role of habits in supporting the performance of repeated behaviors through remembering.