Emissions from airplanes and their potential global effects on the atmosphere have become the subject of intensive study by scientists, and are now drawing the interest of governments. Global fuel consumption has risen much faster for aviation than for other energy-use sectors. Concerns have focused on the contribution of nitrogen oxides (NOx), carbon dioxide (C02), water vapor (H20) and other engine effluents to the buildup of the atmosphere's greenhouse effect. Future aircraft emissions also may affect the stratosphere's ozone layer. This report describes an effort to develop long-term scenarios for emissions from aviation in order to provide a basis for assessing their potential environmental impact throughout the 21st century. Carbon dioxide and nitrogen oxides from the current and projected subsonic aircraft fleets are the main focus of this study. The scenarios in this report were produced by a model that builds on technological and operational assumptions made by industry and government for the period through 2015. It is important to state from the outset what this report is not about. It is not a detailed examination of the environmental effects of aviation. It is not an assessment of the potential for technological or operational changes that could reduce emissions from expected levels. It does not set forth a comprehensive and detailed policy prescription for limiting emissions from aviation. This report does not analyze the potential emissions of a vastly expanded fleet of supersonic aircraft, such as the proposed High-Speed Civil Transport (HSCT), although its possible environmental impacts are discussed briefly.