The knowledge of indoor air humidity in the design phase is important to decide on the appropriate moisture control measures to prevent moisture problems in building components. Because of the uncertain nature of most of the factors affecting the indoor humidity, its accurate prediction in the design phase is not possible. To overcome this problem, the concept of Indoor Climate Classes has been introduced and used in Europe since its early development in the Netherlands in the 1970s up to its recent introduction in a European Standard on the hygrothermal performance of building components. The concept allows to make a rough quantification of the indoor humidity on the basis of building type and function in order to evaluate the moisture performance of building components in the design phase. The first part of the paper gives an introduction to the concept of indoor climate classes and reviews the methodologies to define the boundaries between different classes. The second part of the paper presents a model to predict the transient indoor humidity, based on a physical analysis of the mass balance of an enclosure (vapor production, ventilation and moisture storage in building materials). The model is applied to discuss the different factors affecting the humidity in a room and to guide the classification of humidity in dwellings.