In the framework of a project of the International Energy Agency (IEA) , IEA-Annex XVIII - Demand Controlled Ventilating (DCV) Systems, which started in fall 1987, a review of the state of the art of already existing DCV systems and devices has been undertaken by all participating countries. This paper is concerned with air quality sensors which may be suitable to control air quality on demand. The dominant contaminants are not only variing in different kinds of buildings (dwellings, schools, stores etc.) but also from room to room due to different ways of utilizing the spaces. Climatic and environmental differences will have a further impact on the DCV system. In this context contaminants are discussed which have a dominant regime and impact on indoor air quality and which cannot be avoided by controlling the source. These are humidity, odours (indicator e.g. carbon dioxide CO2), fumes, and tobacco smoke. The working principles of various sensors are outlined and possibilities of application discussed.