Self-determination theory (SDT) is a macro theory of human motivation and personality that concerns people's innate growth tendencies and innate psychological needs. It pertains to the motivation behind people's choices in the absence of external influences and distractions. SDT focuses on the degree to which human behavior is self-motivated and self-determined.

In the 1970s, research on SDT evolved from studies comparing intrinsic and extrinsic motives, and from growing understanding of the dominant role that intrinsic motivation played in individual behavior. It was not until the mid-1980s, when Edward L. Deci and Richard Ryan wrote a book titled Intrinsic Motivation and Self-Determination in Human Behavior, that SDT was formally introduced and accepted as a sound empirical theory. Since the 2000s, research into practical applications of SDT has increased significantly.

The key research that led to the emergence of SDT included research on intrinsic motivation. Intrinsic motivation refers to initiating an activity because it is interesting and satisfying in itself to do so, as opposed to doing an activity for the purpose of obtaining an external goal (extrinsic motivation). A taxonomy of motivations has been described based on the degree to which they are internalized. Internalization refers to the active attempt to transform an extrinsic motive into personally endorsed values and thus assimilate behavioral regulations that were originally external.

Edward Deci and Richard Ryan later expanded on the early work differentiating between intrinsic and extrinsic motivation and proposed three main intrinsic needs involved in self-determination. According to Deci and Ryan, three basic psychological needs motivate self-initiated behavior and specify essential nutrients for individual psychological health and well-being. These needs are said to be the universal and innate need for autonomy, competence, and relatedness.