Chapters 3 and 4 described the implementation of a GUI for digital circuit layout and a simulator engine for digital circuits, respectively. These two software entities can each operate as stand-alone applications; unfortunately, they are each limited in their functionality. The GUI layout application has no integrated simulation capabilities and the simulator engine has a very primitive interface by which users must tediously describe the circuits and their inputs textually and then decipher the textual signal outputs after simulation. This chapter describes how these two separate entities have been integrated together to form a single application from the perspective of the end user, thereby providing the benefits of both software units. The advantages of this highly modular approach towards system integration will also be discussed.