In this paper we describe a particular approach to specify properties of software systems using attributed graphs. The necessary change, dynamics etc. in such systems is defined by rewriting graphs in a structured and abstract way. Since graphs are an intuitive way to define system properties statically, graph rewrite rules in form of graph grammars define modification of graphs formally. In order to structure the otherwise inherent flat graphs a module concept for graph grammars is gradually motivated and defined. We show the applicability of our approach to modularizing graphs and graph grammars by discussing two problems from GUI-specification and formalizing software architecture, respectively.