Three basic questions are frequently asked in the context of psychiatry and psychology: What kinds of behavior are judged to be abnormal, whether by professionals or lay persons? What are the various patterns or forms of disturbed behavior? How can one make sense of the apparently senseless or irrational behavior of disturbed persons? For existential-phenomenological psychology, which seeks to understand human existence on its own terms and to disclose the significance of behavior for the actor as well as the observer, the last question is the most fundamental and constitutes the major focus of this chapter.