The purpose of high-fidelity modeling is to identify the essential software elements, their behavior, and their relations alongside a verified source code base that represents the semantics of that model. This chapter describes the agile best practices applied in the Harmony process for simultaneously developing consistent and defect-free models and source code. These best practices include high-fidelity modeling, test- driven development, and continuous integration. A short discussion of high-fidelity modeling for the C language is provided as well. Exercises are provided for high-fidelity modeling for the Roadrunner Traffic Light Control System and the Coyote Unmanned Aerialir Vehicle.