In this chapter we describe Personality Systems Interaction theory (PSI theory) and elaborate how it integrates different theories of personality into a hierarchy of seven levels of personality functioning. Second, we outline PSI theory's dynamic approach to personality by describing the personality systems interactions underlying the two pivotal personal competences of decisiveness and self-growth. Third, we will discuss experimental findings testing crucial assumptions concerning personality systems interactions. Finally, we conclude with a brief discussion of some applications of PSI theory to personality development and psychological intervention.