This talk will discuss the role of diagrams in geometric proofs.  I will discuss some of the history of the use of diagrams in proof, leading up to a twentieth century bias against the use of diagrams.  More recent work reveals that we can understand precise rules by which diagrams can be used in geometric proofs, and even formalized.  We will discuss these rules, and I will demonstrate my computer program CDEG (“Computerized Diagramatic Euclidean Geometry”) for giving completely formal diagrammatic proofs.  Understanding the rules by which diagrams can be used in geometry proofs has important implications for how we teach proof in geometry classes and how we help prospective teachers think about doing proofs with their own students.