The paper discusses air cleaning for acceptable indoor air quality. Air cleaning has performed an important role in heating, ventilation, and air-conditioning systems for many years. Air cleaning is suggested as a means to reduce the need for fresh air to meet ventilation requirements. Critical to the understanding and application of air cleaners are scientifically valid ways of testing performance. Experimental techniques have been developed to evaluate in-duct air cleaners for both particles and organic compounds. Laboratory performance data of a number of air cleaners will be presented. Building simulation models including laboratory performance data have been used to estimate the importance of air cleaners from a building system standpoint.