A major contributor to the development of a quality final product is a complete, consistent, and detailed requirement specification (Pressman, 2000). No matter how good the specification and its translation into an initial system, it will evolve once it is released to users as the requirements and the environment change and the users develop. The aim of this chapter is to provide a method of establishing the baseline in terms of the requirements of a system from which evolution metrics can be effectively applied. UML (Rumbaugh, Jacobson, & Booch, 1999) provides a series of models that can be used to develop a specification which will provide the basis of the baseline system. This can then be used as a datum from which measurements can be made. One of the starting points for modeling is use case analysis. Other models can then be developed based on these initial models. One of the difficulties with this approach is that once the initial models have been agreed upon, they are not maintained as the later more detailed models evolve. The methods described in this chapter show how this can be achieved and measured.