Self-determination theory (SDT) posits 3 evolved psychological needs, for autonomy, competence, and relatedness. Considerable research has established that all 3 experiences are important for well-being. However, no SDT research has examined whether unmet needs have motivational force, an important criterion for establishing that certain experiences are indeed basic needs and motives (R. F. Baumeister & M. R. Leary, 1995). Three studies, using cross-sectional, experimental, and longitudinal methodologies, supply evidence that felt deficits in autonomy, competence, and relatedness arouse corresponding desires to acquire the missing experiences. However, a positive surfeit of felt-need satisfaction did not predict reduced desires for the corresponding needs. Implications for homeostatic, evolutionary, and humanistic perspectives upon basic psychological needs are discussed.