Henry Stack-Sullivan, who is considered the father of interpersonal psychiatry, developed the Interpersonal Theory of Nursing. This theory explained the role of interpersonal relationships and social experiences in regards to the shaping of personalities, as well as the importance of life events to psychopathology. Stack-Sullivan's theory states that the purpose of behavior is for the patient to have his or her needs met through interpersonal interactions, as well as decrease or avoid anxiety.

The Interpersonal Theory explains six developmental stages, which Stack-Sullivan calls "epochs" or heuristic stages in development.

The first stage, called Infancy, occurs from birth to eighteen months. The main characteristic of this stage is the gratification of needs. The second stage begins at eighteen months and runs until six years of age. This stage, Childhood, is characterized by delayed gratification. The Juvenile Era, which occurs between six and nine years of age, is characterized by the formation of a peer group. The fourth stage, Preadolescence, is between nine and twelve years of age. It is characterized by the development of relationships within the same gender. Early adolescence occurs from twelve to fourteen years. During this stage, the adolescent develops an identity. The sixth stage, late adolescence, runs from fourteen to twenty-one years of age. This final stage in Stack-Sullivan's model of nursing is characterized by the formation of lasting, intimate relationships.

The Interpersonal Theory explains three types of self: the good me, bad me, and not me. The "good me" versus the "bad me" based on social appraisal and the anxiety that results from negative feedback. The "not me" refers to the unknown, repressed component of the self.

Stack-Sullivan's theory also explains anxiety, self-system, and self-esteem. It states that security operations are those measures that the individual employs to reduce anxiety and enhance security. A self-system is all of the security operations an individual uses to defend against anxiety and ensure self-esteem.

This model of nursing provides the basis for interpersonal psychotherapy to specifically address patients with depression and schizophrenia. The theory proposes that depression most often develops in the context of adverse events, especially loss.