The paper summarizes material presented at ASHRAE's Indoor Air Quality (IAQ) 1988 meeting in Atlanta, Georgia in April 1988. One conclusion drawn from several papers presented at the meeting, focusing on the role of particulate and gaseous air cleaners in improving IAQ, is that air cleaners have an important role in improving IAQ. However, they must be designed, operated, and maintained properly. There are two main classes of indoor air cleaners: in-duct and room cleaners. In-duct cleaners are installed within heating, ventilating, and air conditioning (HVAC) systems, and depend on the HVAC fans, ducts, etc. to move dirty air into the air cleaner and cleaned air back into the building. Room air cleaners are self-contained, and designed to handle the air in a single room. Particles in both types of cleaners are collected by fibrous filtration, electrostatic precipitation, or electrostatically augmented filtration. Organic vapors are usually adsorbed by activated carbon filters which can be installed in either type of cleaner. Although many comments in the paper apply to both types of cleaners, it focuses on in-duct units. The rest of the paper discusses factors that affect air cleaner effectiveness, data on particulate and organic cleaners, and measurement techniques.