The Department of the Navy (DON) has launched its Next Generation Enterprise Network (NGEN) program to replace the Navy Marine Corps Intranet (NMCI) program. NGEN capabilities, such as secure transport of voice and data, data storage, and e-mail, are to be incrementally acquired through multiple providers. As planned, the first increment is expected to provide comparable NMCI capabilities, additional information assurance, and greater DON network control, at a cost of about $50 billion through fiscal year 2025. Given the size, importance, and complexity of NGEN, GAO was asked to determine whether DON has sufficiently analyzed alternative acquisition approaches and has a reliable schedule for executing the program, and whether program acquisition decisions have been performance- and risk-based. To do this, GAO reviewed the NGEN analysis of alternatives, integrated master schedule, and key milestone decisions. GAO is recommending that DOD limit further investment in NGEN until it conducts an interim review to reconsider the selected acquisition approach and addresses issues discussed in this report. In its comments, DOD stated that it did not concur with the recommendation to reconsider its acquisition approach; GAO maintains that without doing so, DOD cannot be sure it is pursuing the most cost-effective approach.