The therapeutic alliance has often been found to explain significant portions of the variance in therapy outcome, yet surprisingly little is known about the participant factors that contribute to the development of a strong working alliance. One participant factor that may play a key role in the development of the alliance is attachment. In the present paper, research is reviewed that relates to the influence of therapist and client attachment on alliance, and an investigation is presented that sought to further address this question in a sample of 4 therapists and 69 clients from a randomized controlled trial of cognitive-behavioral therapy (CBT) for generalized anxiety disorder (GAD). Results provide support for the influence of attachment factors on the development of the alliance. Specifically, therapist attachment significantly influenced both early client-rated working alliance, and the trajectory of alliance development over time. Client attachment factors did not influence alliance ratings early in therapy, but two dimensions of attachment, vulnerability and lacking in childhood memories, significantly influenced the development of alliance over time. Implications for future research and clinical applications are discussed.