The economic and technical success of historic building conservation should be based on a comprehensive evaluation of the building's actual condition and its ability to continue to function or to meet the needs of a new program. Too often unnecessary work is undertaken in the name of restoration or rehabilitation without understanding the economic, social, and cultural values of the existing architectural character and fabric. The application of available knowledge and technology to systematically diagnose a building's condition or diseases and make a prognosis as well as prescribe treatment is the basis of an analytical approach that will promote a building's longevity. There are new opportunities for applied research to utilize existing technology from material sciences or other technical fields for the diagnosis and monitoring of conditions of building materials and systems. The importance of understanding the principles of existing building performance, the practical development of diagnostic skills, and an understanding of the purpose, range, and potential of building diagnostic techniques are discussed.