The actor-partner interdependence model (APIM) is a model of dyadic relationships that integrates a conceptual view of interdependence with the appropriate statistical techniques for measuring and testing it. In this article we present the APIM as a general, longitudinal model for measuring bidirectional effects in interpersonal relationships. We also present three different approaches to testing the model. The statistical analysis of the APIM is illustrated using longitudinal data on relationship specific attachment security from 203 mother-adolescent dyads. The results support the view that interpersonal influence on attachment security is bidirectional. Moreover, consistent with a hypothesis from attachment theory, the degree to which a child's attachment security is influenced by his or her primary caregiver is found to diminish with age. (Contains 2 tables, 2 figures, and 4 footnotes.)