This book describes the application of simulation techniques to the operational test and evaluation (OT&E) of command support systems (CSS). OT&E is essential for ensuring that systems are effective, suitable for service and meet the end users' needs. Simulation can be used in OT&E to simulate entities that cannot be obtained for real-world OT&E and to evaluate the effectiveness of systems in controlled environments. This book defines the relationships between command and control (C2), CSS, systems engineering, verification and validation, OT&E and modelling and simulation. This book describes how simulation techniques have been used to improve OT&E of CSS and how simulation models are verified, validated and accredited. This book describes how the effectiveness of an OT&E program can be measured in terms of its cost effectiveness with respect to gathering evidence to support the resolution of critical operational issues (COIs). This book describes a proposed methodology for applying simulation in OT&E of CSS to help gather data to resolve COIs when it is impractical to conduct sufficient real-world OT&E to gather sufficient data to resolve COIs.