Theories are among the main achievements of scientific inquiry and appear as the repositories of scientific knowledge. This chapter is devoted to an examination of the notion of theory as a unit of analysis for the study of scientific knowledge. Most of the analysis consists in presenting and criticizing two major proposals made by philosophers of science, the “received view,” commonly attributed to logical empiricism, and the “semantic view of theories,” which became the new orthodoxy in the 1960s. Both proposals aim at formal reconstructions of theories. The shared assumptions underlying this common project will be questioned. Alternative ways of construing scientific theorizing will be sketched, notably those which are more “agent-centered” and put forward the way scientists use and understand their theories in practice.