CDEG (Computerized Diagrammatic Euclidean Geometry) is a computer proof system in which diagrammatic proofs of theorems of Euclidean Geometry can be given formally. The computer system manipulates geometric diagrams using an internal representation that is based on the idea that all the significant information in a geometric diagram is captured by its underlying topology. The proof system that CDEG implements is that of the author’s diagrammatic formal system for geometry, FG. CDEG and FG are strong enough to be able to duplicate most, if not all, of the proofs in the first several books of Euclid’s Elements. This paper explains CDEG and gives a brief example of how it works.