Specialized psychotherapy for deaf people in the Dutch and Western European mental health systems is still a rather young specialism. A key policy principle in Dutch mental health care for the deaf is that they should receive treatment in the language most accessible to them, which is usually Dutch Sign Language (Nederlandse Gebarentaal or NGT). Although psychotherapists for the deaf are trained to use sign language, situations will always arise in which a sign language interpreter is needed. Most psychotherapists have the opinion that working with a sign language interpreter in therapy sessions can be a valuable alternative option but also see it as a second-best solution because of its impact on the therapeutic process. This paper describes the authors' years of collaboration as a therapist and a sign language interpreter. If this collaborations is optimal, it can generate a certain 'therapeutic power' in the therapy sessions. Achieving this depends largely on the interplay between the therapist and the interpreter, which in the authors' case is the result of literature research and our experiences during the last 17 years. This article analyzes this special collaborative relationship, which has several dimensions and recurrent themes like, the role conception of the interpreter, situational interpreting, organizing the interpretation setting, or managing therapeutic phenomena during therapy sessions.