Project 25 (P25) is a standards development process for the design, manufacture and evaluation of interoperable digital two-way wireless communications products created by and for public safety professionals. The published P25 standards suite is administered by the Telecommunications Industry Association (TIA Mobile and Personal Private Radio Standards Committee TR-8). Radio equipment that demonstrates compliance with P25 should be able to meet a set of minimum requirements to fit certain needs of public safety, including interoperability, allowing users on different systems to talk via direct radio contact. Access to P25 CAP laboratory recognition is available to public and private testing laboratories, including commercial laboratories, equipment manufacturers' in-house laboratories, university laboratories, and federal, state, and local government laboratories from inside or outside the United States. This handbook is sponsored by the NIST Office of Law Enforcement Standards (OLES) Public Safety Communications Systems program, which provides technical expertise to the DHS Office of Interoperability and Compatibility that is building a framework for fully interoperable communications among all first response agencies. It is also developing standards for voice, data and image transfers, evaluating existing devices and services, and developing a uniform assessment process for testing the conformity of mobile radio equipment with P25 requirements.