It used to be said that models were dispensable aids to formulating and understanding scientific theories, perhaps even props for poor thinkers. This negative view of the cognitive value of models in science contrasts with today's view that they are an essential part of the development of theories, and [Page 827]more besides. Contemporary studies of scientific practice make it clear that models play genuine and indispensable cognitive roles in science, providing a basis for scientific reasoning. This entry describes types and functions of models commonly used in scientific research.