Book excerpt: The Next Generation Air Transportation System (NextGen) is an effort begun in 2003 whose goals include improving the capacity, efficiency, and safety of the U.S. air transportation system and also enabling reduction in noise, pollution, and energy use. The Federal Aviation Administration and various stakeholders, including equipment providers, airlines, and contractors, are currently implementing both near-term and midterm capabilities of this effort. Interim Report of a Review of the Next Generation Air Transportation System Enterprise Architecture, Software, Safety, and Human Factors is part of a larger project to examine NextGen's enterprise architecture and related issues. This interim report provides an initial assessment focusing on challenges of system architecture for software-intensive systems.