From our earliest practice of family therapy at medical schools, private family therapy institutes, and public agencies, our work with difficult populations that do not respond to current treatment technologies has reminded us of the inadequacies of our theoretical descriptions and the limitations of our expertise. This work has influenced our current, evolving clinical theory as we move from thinking of human systems as social systems defined by social organization (role and structure) to thinking of them as distinguished on the basis of linguistic and communicative markers. Hence, for us, the social unit we work with in therapy is a linguistic system distinguished by those who are "in language" about a problem, rather than by arbitrary and predetermined concepts of social organization. We call the therapy system a problem-organizing, problem-dis-solving system.