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Good ventilation is critical for housed dairy cattle during hot weather. Three ventilation parameters rate of air exchange, air distribution, and air velocity at animal level need to be considered in assessing ventilation. This paper describes each parameter, discusses relevant research, and points out deficiencies in the existing information pool. Research is needed to establish the minimum air-exchange rate during hot weather for modern dairy cattle. Recommended rates are likely being attained in modern naturally ventilated and tunnel-ventilated barns for most situations. Exceptions would include naturally ventilated barns that are poorly sited and/or oriented, very wide and/or densely stocked, or located in a region that experiences a significant number of summer days with calm or low-wind conditions; as well as tunnel-ventilated barns that are longer or more densely stocked than is warranted based on fan and inlet capacity. Proper air distribution presents a challenge for natural ventilation in terms of site planning and for tunnel ventilation in terms of maintaining airflow through occupied areas. Achieving desired air velocity at cow level in the presence of adequate air exchange appears to be the key design challenge. A variety of approaches are being used to move air within occupied areas to enhance evaporative cooling and maintain thermal comfort. While these approaches have been shown to produce desirable interior airflow, further study is needed to determine the most effective and economically advantageous methods.