Models and simulations (M&S) will be employed to support important design decisions and verification of system requirements in the development of NASA's Orion Crew Exploration Vehicle. Most simulations are implemented in software. For developed software, NASA's software engineering procedural guideline NPR 7150.2 and safety standard NASA-STD-8719.13B apply. Recognizing the need for critical M&S to be validated to be credible for their intended uses, NASA developed a Modeling and Simulation Standard, NASA-STD-70091. This paper analyzes the requirements specified by these standards and their role in test, validation and certification of modeling and simulation software. It discusses simulation validation as a distinct instance of software validation with corresponding unique requirements. Simulation-specific validation concerns include fit to intended use, validation against experimental data, uncertainty quantification, and sensitivity analysis. The paper also describes the Orion M&S verification, validation, and accreditation (VV&A) process.