PROforma is a formal language for modeling clinical processes that was developed by the Advanced Computation Laboratory of Cancer Research UK in 1996, together with associated tools, for creating decision support, care planning, clinical workflow management and other applications. The technology has been used to develop and evaluate a number of decision support applications in range of clinical settings. Clinical trials have been carried out and published for seven of these applications, all suggesting major positive benefits on a variety of outcome measures. The most recent and ongoing project called CREDO is an ambitious attempt to address the challenges in deploying sophisticated decision support systems in the intricate and convoluted management of chronic diseases, taking breast cancer as an example. In this chapter we describe the implementation of evidence based clinical guidelines within a complex care-pathway for patients with breast symptoms and analyse in detail the results of an evaluation study. Some important lessons learned during the process are shared and future directions are discussed.