The view that knowledge cannot be transmitted but must be constructed by the mental activity of learners underpins contemporary perspectives on science education. This article, which presents a theoretical perspective on teaching and learning science in the social setting of classrooms, is informed by a view of scientific knowledge as socially constructed and by a perspective on the learning of science as knowledge construction involving both individual and social processes. First, we present an overview of the nature of scientific knowledge. We then describe two major traditions in explaining the process of learning science: personal and social constructivism. Finally, we illustrate how both personal and social perspectives on learning, as well as perspectives on the nature of the scientific knowledge to be learned, are necessary in interpreting science learning in formal settings.