The museum Kunstforum Ostdeutsche Galerie in Regensburg participated, with international partners, in the EUREKA-Projekt 1383 PREVENT, which was concerned with climate in museums and historical buildings. During the Museum’s renovation from 1989 to 1993, an alternative climate concept was realized which can be characterized by the key words “optimized building envelope / wall tempering / minimized ventilation”. Long-term climate measurements as part of the PREVENT Project demonstrated that this concept produces very good, stable climate conditions. The restorer must know to which technical consequences his demands lead in practice and which options are available. Until now, complete central air conditioning systems, which perform all climate control tasks: heating, cooling, humidifying, dehumidifying, ventilating, and fresh air exchange together in one central unit, were the “solution” to all climate control problems. However, a detailed reexamination of each of these individual tasks shows that such systems are rather the “problem”; that, in general, air movement is bad, air is the wrong medium for heating, and it is impossible to centrally perform all climate control tasks optimally (or for museums even adequately). The preventive quality of buildings and furnishings must be included in designing climate control systems, so that as little treatment of room air as possible is needed. Complete central air systems not only make little sense but are expensive to purchase and maintain. Optimized building envelopes with wall tempering and minimized ventilation have conservational, physiological, energetic, and cost advantages. This article also considers development of the so-called international climate values for museums. According to a literature study by Holmberg, the rigid demand for maintaining 55% [plus minus] 5% relative air humidity should not be seen as a standard. A broad discussion based on relevant recent literature must be initiated. A new American Society of Heating, Refrigeration, and Air Conditioning Engineers standard for museums, libraries, and archives has been published, which takes short-term fluctuations and seasonal variations into account. Instead of the previous rigid values, specific fluctuation control levels are to be maintained.