Self-determination theory (SDT) is an empirically based, organismic theory of human behavior and personality development. SDT’s analysis is focused primarily at the psychological level, and it differentiates types of motivation along a continuum from controlled to autonomous. The theory is particularly concerned with how social-contextual factors support or thwart people’s thriving through the satisfaction of their basic psychological needs for competence, relatedness, and autonomy. Although the theory is psychological, research has also given attention to biological underpinnings of these psychological processes and places them in an evolutionary perspective. In this chapter we provide an overview of what appears in the chapters that follow, including a layout of SDT’s six mini-theories; a discussion of a range of phenomena related to human development; an argument for the theory’s applicability to real-life domains such as education, health care, work, psychotherapy, sport, and virtual worlds; and a consideration of social, political, and cultural factors that influence motivations and basic need satisfactions.