Graph Theory, which studies the properties of graphs, has been widely accepted as a core subject in the knowledge of computer scientists. So is Object-Oriented (OO) software engineering, which deals with the analysis, design and implementation of systems employing classes as modules. The latter field can greatly benefit from the application of Graph Theory, since the main mode of representation, namely the class diagram, is essentially a directed graph. The study of graph properties can be valuable in many ways for understanding the characteristics of the underlying software systems. Representative examples for the usefulness of graph theory on OO systems based on recent research results are presented in this paper.