GAO reviewed the Civil Reserve Air Fleet (CRAF) program to determine whether: (1) the Department of Defense's (DOD) efforts to ensure that CRAF was ready for mobilization were sufficient and effective; and (2) commercial carriers were prepared to support CRAF aircraft, particularly at foreign airfields. GAO found that it was uncertain whether CRAF could effectively meet DOD mobilization requirements because: (1) Military Airlift Command (MAC) tests of the program through simulation and field exercises were very limited; (2) DOD had provided limited mobilization planning data to CRAF carriers, making it very difficult for the carriers to plan for utilization of the system in an emergency; (3) MAC had not sufficiently monitored carrier compliance with contract provisions designed to help ensure effective mobilization; and (4) the incompatibility of data communications services at some military airfields with existing commercial services could hinder effective communications. GAO also found that, at overseas airfields, CRAF might not get the support needed because responsible carriers were unaware of the estimated work load for each location. Having allies provide this support under host-nation support agreements is an option that DOD is pursuing.