Concurrency is a significant challenge to the aircraft simulation industry. Concurrency is achieved when the simulator configuration is kept current with, or slightly ahead of, the aircraft configuration. This allows aircrews and maintenance personnel to train on new systems before they experience them on the aircraft. The most important consideration in designing for concurrency is simulating the avionics systems such that future changes to the aircraft configuration and software can be incorporated into the simulator with minimal time and resources. With the proliferation of avionics modernization programs and Global Air Traffic Management (GATM) driven upgrades, concurrency has an even greater significance. The C-130 and C-5 Avionics Modernization Programs (AMP), AH/MH-6M Mission Enhanced Little Bird (MELB) upgrade, H-60R/S Common Cockpit upgrade, and the EA-6B Improved Capabilities III (ICAP3) program are just a few current examples. This paper describes one of the primary tools available to help achieve concurrency objectives - ARINC Report 610, Guidance for Use of Avionics Equipment and Software in Simulators. Although a commercial document, ARINC Report 610 can be effectively applied to military programs. There is no equivalent military guidance document. This paper describes the history of ARINC Report 610, from its initial publication in 1987, through the release of revision B in December 2001. The rationale behind the A and B revisions is described, and supplemental guidance information is provided. Finally, the paper suggests how to use ARINC Report 610 on military simulator development programs.