Public understanding of science and public engagement around science policy issues takes place within a complex system involving public opinion, values, identities, social groups, media, and social, economic, and political structures and influences. This paper offers a theory of agency that bridges the psychological and sociological and thereby offers a theoretical framework for understanding the system underlying public understanding of science, support for science, and the implications of public engagement. Agency theory integrates elements of well-established social science theories into an understanding of human agency. Strategies for improved public engagement are discussed.