Motivation is the largest single topic in psychology, with at least 32 theories that attempt to explain why people are or are not motivated to achieve. Within sport psychology research, there are a plethora of techniques of how to increase and sustain motivation (strategies to enhance agency beliefs, self-regulation, goal setting, and others). However, when explaining the conceptual undergirding of motivation in sport, the why of motivation, two theories predominate: Achievement Goal Theory (AGT) and Self-Determination Theory (SDT). Both theories predict the same outcomes, such as increased achievement striving, sustained behavior change, and perceptions of well-being, but they differ in why those outcomes occur. AGT assumes that individuals cognitively evaluate the competence demands and meaningfulness of the activity, and that those perceptions govern behavior. SDT assumes that individuals are driven by three basic needs, competence, autonomy, and relatedness, and the satisfaction of those needs govern behavior. The following discusses both theories and concludes that each has their strengths and weaknesses.