The paper deals with transactionality, i.e. the mutual intertwining of defensive functions in lasting and intimate, though conflictual, dyads, groups and families. From the individual point of view the concept refers to the exclusive use of those defensive functions through which the person tries to protect himself against anxiety by drawing on other persons and/or fantasies about them. Genetically, transactionality of defensive processes may be either regressive, due to fixation at the last stages of self-object differentiation, or indicate a true developmental arrest. From the therapeutic point of view the role of separation anxiety appears most crucial. Therapeutically, it is important to focus on the delayed mourning so often behind commonly held defensive interactional positions.