This one-day workshop is designed to integrate a wide variety of cognitive science perspectives on the roles diagrams play in cognition, addressing various ways in which people design and use diagrams to spatialize thought and make it public, to work through ideas and clarify thinking, to reduce working memory load, to communicate ideas to others, to promote collaborative work by providing an external representation that can be pointed to and animated by gestures and collectively revised. The morning session (talks by Tversky, Healey, and Kirsh) will focus on creating and coordinating activities with diagrams, the afternoon (talks by Bechtel, Cheng, and Hegarty) will examine uses of diagrams in science. Both session will also include blitz talks presenting one major idea; scholars who would like to present blitz talks should contact the session organizer, William Bechtel.