ABSTRACT To discern what accounts for moment-to-moment fluctuations in well-being, the present study investigated how state-level autonomy relates to three aspects of well-being: affect, engagement, and meaning measured at the momentary level using the experience sampling method (ESM). These effects were contrasted with the impact of activity types (work, study, play, rest), controlling for life satisfaction and demographic differences, using multilevel regression analyses. Controlling for all other predictors, autonomy was the only significant predictor for momentary affect and engagement; it was the strongest predictor for momentary meaningfulness. Autonomy showed a positive linear relationship in predicting affect, whereas the relationship was quadratic for the remaining two aspects of well-being such that engagement and meaning increase as autonomy rises from none, low, to moderate levels but plateau from moderate to high autonomy. Results suggest that beyond what people do, a key to well-being may be experiencing higher autonomy without necessarily eliminating extrinsic motivation.