Notes that over the last 15 years, active desiccant systems have become a common component of HVAC systems in commercial buildings needing lower than usual humidity levels. Ice rinks, supermarkets and refrigerated warehouses all contain refrigeration systems which cool air more effectively when most of the building's moisture load is removed by an active (heat reactivated) desiccant system Cost savings, comfort improvements and 'process benefits' of extended season operation for ice rinks, lower product temperature for supermarkets and improved safety for warehouses are usually enough to make the desiccant component a useful addition. Notes that more recently active desiccant systems have been applied to ventilation systems of buildings with no obvious need for low humidity, such as schools, retail stores, restaurants, hotels, cinemas and care facilities for the elderly. Discusses active and passive desiccant systems of pretreating ventilation air and illustrates an active system diagrammatically. Describes in some detail their operation and calculation of costs vs. comfort for these applications, based on cinemas in various locations in the US.