ABSTRACT Interest in intra-individual variation in trait expression across situations, contexts, and relationships, and the meaning of this variation for personal functioning has grown significantly. In this article we review this literature with an emphasis on (a) appropriate methods for identifying variations in trait expression and (b) the substantive meaning and sources of this variation. Self-determination theory suggests that people will express traits differently as a function of the degree of support for autonomy they experience in any given setting. Accordingly, autonomy support is shown to predict variations in Big Five trait expression and other stable individual differences such as attachment security and dependency. The discussion focuses on methodological issues in the study of variability and on why autonomy support may play a central role in explaining trait variability and its relation to well-being.