This chapter covers state-of-the-art specification languages, tools, and methodologies for processor development in academia as well as industry. Time-to-market pressure coupled with short product lifetimes creates a critical need for design automation in processor development. The processor is modeled using a specification language such as Architecture description language (ADL). The ADL specification is used to generate various tools (e.g., simulators, compilers, and debuggers) to enable exploration and validation of candidate architectures. The goal is to find the best possible processor architecture for the given set of application programs under various design constraints such as cost, area, power, and performance. The ADL specification is also used to perform various design automation tasks, including hardware generation and functional verification of processors.