This paper introduces a theoretical system within which relating behaviour may be defined, classified and measured. It is emphasized that a person both relates to and is related to by others. Relating may be of a positive or negative kind, and it is argued that the object of psychotherapy is both to eliminate the person's negative relating and enable the person to tolerate the negative relating of others. The term 'interrelating' is used to refer to the combination of relating and being related to of two people. Couple therapy is aimed at correcting the negative interrelating of partners, and family therapy is aimed at improving the interrelating of pairs of family members. A set of questionnaires based upon the theoretical system is introduced. These measure the negative relating of individuals and negative interrelating between individuals, and are proposed as useful adjuncts to individual, couple and family therapy.