Sex is central to most intimate relationships, and there is inevitably a two-way interaction between sex and intimacy. Sex is a physical, a psychological and an interpersonal event, and treatment of sexual problems should take account of all three aspects. Couple relationship problems that can affect sexual interaction include pervasive hostility, poor communication, lack of trust and disparate levels of sexual desire. Therapists should be able to manage therapy sessions with both partners present, without taking sides, and treating the relationship, rather than either individual, as the ‘client’. They should be able to balance the physical approach to problems with managing relationship issues and the partners' individual psychological problems. Techniques are recommended for understanding and modifying the couple relationship during therapy, as well as for dealing with specific sexual problems using a behavioural systems approach.