Can empirical research guide clinicians in how to conduct effective couple therapy? While we know that couple therapy works, understanding the mechanisms of action-how and why it works-has been the focus of research for several decades. Using Doss's framework for understanding the key components of the change process-therapy change processes, client change processes, mediators, and outcomes-we reviewed 48 quantitative couple therapy process studies over a 40-year period. The results reveal a fragmented knowledge base. No single study examines the entire process of change, and although several findings show promise, none have been replicated. Additionally, only a limited number of hypothesized associations achieved statistical significance, with neither theory nor empirical evidence adequately explaining why some hypotheses were supported while others were not. Current couple therapy process research cannot yet guide clinicians on how to conduct effective therapy. Until it does, the divide between research and practice will persist.