Allison Engine Company has developed and verified key combustion system technologies to satisfy ATS Phase 2 performance requirements. These activities include the following: demonstration test of an ultra-lean premix module meeting the ATS 8 ppm NOx goal using natural gas fuel; design and fabrication of a second generation premix module for bench test evaluation; bench test verification of catalytically enhanced combustion; and preliminary design of the transition section that guides the combustor discharge flow from the external combustor to the turbine inlet. Allison has been executing a systematic approach in developing the combustion system technologies to insure that the ATS engine includes the benefits of advanced material and low NOx combustion technologies without placing undue risk on the overall engine development program. New technology is most easily assimilated in discrete evolutionary stages; thus Allison has structured the combustion system development plan with a series of increasingly demanding performance evaluations that demonstrate the suitability of the individual technology. The discussion summarizes the progress made in bringing advanced combustion technology to the ATS.