This paper focuses on the contributions of two lifespan-development theories to the study of adult intimate relationships. These are interpersonal acceptance-rejection theory (IPARTheory) and attachment theory. First, we focus on four major points of convergence and four points of divergence between theories in their understanding of intimate relationships in adulthood. Following this, we discuss each theory's contributions to theoretical and empirical knowledge about adult intimate relationships, their modes of assessment, and the development of clinical interventions. Finally, we suggest possible future developments that could help to enrich both theories.