Geant4 exploits advanced software engineering techniques based on the Booch/UML Object Oriented Methodology and follows the evolution of the ESA Software Engineering Standards for the development process. The "spiral" (iterative) approach has been adopted; User Requirements were collected in the initial phase and Problem Domain Decomposition, Object Oriented Methods and CASE tools have been employed for the Analysis and Design, giving a clear hierarchical structure of sub-domains linked by a uni-directional flow of dependencies. This leads to a software product which is modular and flexible (a toolkit) and in which the physics implementation is transparent and open to the user validation of the physics predictions. It allows the user to understand, customise and extend the toolkit in all the domains; at the same time the modular architecture allows the user to load only needed components.