Treatment-interfering behaviors is a term introduced by Marsha Linehan (1993) to describe behaviors that clients bring to bear within and between sessions and which impede the progress of therapy. Common treatment-interfering behaviors include failure to attend sessions consistently, failure to keep to contracted agreements, arguing with the therapist, refusal to engage in conversation, and behavior that oversteps therapist limits. While this designation gives a name to a class of behaviors that are problematic in therapy, Linehan’s designation focuses entirely on the client. However, clinical observation suggests that besides clients, other factors can and do impede treatment progress. Accordingly, this book adopts a broader designation: therapy-interfering factors, which include client behaviors but also recognize the influences of the therapist, client-therapist relationship, and intervention factors as impediments to treatment progress.