Aviation is an integral part of the global transportation network, and the number of flights worldwide is expected to grow rapidly in the coming decades. Yet, the effects that subsonic aircraft emissions may be having upon atmospheric composition and climate are not fully understood. To study such issues, NASA sponsors the Atmospheric Effects of Aviation Program (AEAP). The NRC Panel on Atmospheric Effects of Aviation is charged to evaluate AEAP, and in this report, the panel is focusing on the subsonic assessment (SASS) component of the program. This evaluation of SASS/AEAP was based on the report Atmospheric Effects of Subsonic Aircraft: Interim Assessment Report of the Advanced Sub-sonic Technology Program, on a strategic plan developed by SASS managers, and on other relevant documents.