Systems Theory can be summed up by the phrase "No man is an island." We belong to many systems, which we effect and in turn effect us. Here's an example of how this applies to therapy:
A number of years ago, researchers were studying children who were admitted to the hospital with asthma. They conducted a study in which the parents were not allowed to visit the children while they were in the hospital. For many of these children, the symptoms cleared up rather quickly; and once they were stabilized, the parents were invited to come back and visit. Things seemed to go along fine, until the parents would get into an argument. As soon as the parents started to fight, the asthma would return. They would then stop fighting, and direct their attention to their child.
From studies like this, people began to look beyond the individual to the roles that she plays in her family. In this situation, the child has the job of keeping the family together by making sure things don't get too overheated between the parents. The child is carrying the symptoms for the family unit; she is the "identified patient." From this perspective, what is needed is not more asthma medication, but marriage counseling for the parents.
One particular application of Systems Theory is Family Systems Therapy. This can be used with families, and also for individuals and couples. For individuals, the person may explore the rules of their family of origin, their role in it, and how that impacts their current relationships. This may be done through techniques such as creating a multi-generation genogram which explores the functioning of a family as a whole; conducting interviews with the parents (when possible); and boundary setting exercises to help the person shift dysfunctional family patterning.
Family Systems Therapy has a positive view of both the individual and the family. As family therapist Salvadore Minuchin said: "When families come to me for help, I assume they have problems not because there is something inherently wrong with them, but because they've gotten stuck - stuck with a structure whose time has passed, and stuck with a story that doesn't work."