This two-part report explores the efforts of businesses and individuals to improve the thermal performance of windows. Part I discusses the basics of what makes a window product insulate or save energy. Topic areas addressed include saving energy lost through windows, key components of window insulation, three basic types of window insulation, amount of energy window insulation can save, and ways to improve the performance of existing window treatments. Part II discusses whether or not to put insulation inside or outside a window, which sealing systems are available and make the most sense to use, and how to operate products used on heat-gaining windows. In addition, choices for new construction and retrofit are explained. Information on how to calculate heat losses through windows, a reading and resource list, and descriptions of Department of Energy appropriate technology grants (reviewed in researching and preparing this document) are included in appendices. (ML)