Ventilation rates have significant impacts on building energy use and indoor contaminant concentrations, making them key parameters in evaluating building performance. Ventilation rates have been measured in buildings for many decades and there are mature measurement approaches available to researchers and others who need to know actual ventilation rates in buildings. Despite the fact that ventilation rates are critical in interpreting indoor concentration measurements it is disconcerting how few indoor air quality (IAQ) field studies measure ventilation rates or otherwise characterize how the building(s) being studied is designed to be and actually is being ventilated. This paper summarizes parameters of interest in characterizing building ventilation, available methods for quantifying these parameters, and the challenges in applying these methods to different types of buildings and ventilation systems. These parameters include whole building air change rates, ventilation system outdoor air intake rates and building infiltration rates. Tracer gas methods are reviewed as well as system airflow rate measurements using, for example, duct traverses. Several field studies of ventilation conducted over the past 75 years are described to highlight the approaches employed used and the findings obtained.