This study explored J. Bowlby's (1988) secure-base hypothesis, which predicts that a client's secure attachment to the therapist, as well as the client's and the therapist's global attachment security, will facilitate in-session exploration. Volunteer clients (N = 59) and trainee counselors (N = 59) in short-term therapy completed the Experiences in Close Relationship Scale (K. A. Brennan, C. L. Clark, & P. R. Shaver, 1998) as a measure of adult global romantic and peer attachment orientations; the Client Attachment to Therapist Scale (B. Mallinckrodt, D. L. Gantt, & H. M. Coble, 1995) as a measure of attachment to counselor; the Working Alliance Inventory (A. O. Horvath & L. Greenberg, 1989) as a measure of working alliance; and the Session Evaluation Questionnaire-Depth Subscale (W. B. Stiles & J. S. Snow, 1984) as a measure of session depth. In line with Bowlby's hypothesis, the findings suggest that session depth is related to the client's experience of attachment security with the counselor and that counselor global attachment moderates the relationship between client global attachment and session exploration. (Contains 4 tables, 1 figure and 1 footnote.)