Windows are significant to the historic character of older buildings, contributing as highly visible features of the exterior and often as distinctive elements of the interior as well. Restoring and rehabilitating windows in historic buildings have become increasingly complex in recent years. Besides the continuing problems of material deterioration and budgetary constraints, the wave of new requirements affecting health, safety, and energy has created enormous challenges. These problems and challenges confront architects, engineers, conservators, and owners of every age and type of historic building. HPEF and its partners, most notably the National Park Service, have produced a variety of programs and publications designed to focus attention on the importance of windows in historic buildings and to provide practical guidance on maintenance, restoration and rehabilitation work. The "Windows Through Time" exhibit is one such activity, documenting the types of windows typically found in American architecture since the eighteenth century. The Window Rehabilitation Guide for Historic Buildings and the Window Workbook are available for download on this site.