Book excerpt: This book describes CoSMoS (Complex Systems Modelling and Simulation), a pattern-based approach to engineering trustworthy simulations that are both scientifically useful to the researcher and scientifically credible to third parties. This approach emphasises three key aspects to this development of a simulation as a scientific instrument: the use of explicit models to capture the scientific domain, the engineered simulation platform, and the experimental results of running simulations; the use of arguments to provide evidence that the scientific instrument is fit for purpose; and the close co-working of domain scientists and simulation software engineers. In Part I the authors provide a managerial overview: the rationale for and benefits of using the CoSMoS approach, and a small worked example to demonstrate it in action. Part II is a catalogue of the core patterns. Part III lists more specific “helper” patterns, showing possible routes to a simulation. Finally Part IV documents CellBranch, a substantial case study developed using the CoSMoS approach.