Using a contemporary Kleinian psychoanalytic model, this paper explores some important theoretical issues relevant to understanding infidelity for the couple psychotherapist. The main focus of this paper is on the problems that one or both of the partners in a couple may have with mourning past or impending loss, and the consequent infidelity that may result from a manic attempt to replace depression or psychic pain with excitement. The concepts of the oedipal situation, triangulation, mourning, and manic defence are outlined in terms of the implications for understanding infidelity. The importance of viewing infidelity as possibly reflecting a range of psychological dilemmas across the developmental spectrum is emphasised. Clinical material is used to illustrate the role of unresolved pre-oedipal and oedipal issues, manic defences, and the ensuing triangulations that may develop, providing the psychic structures for infidelity.