Properly selected and maintained, building air filters provide a simple an often overlooked opportunity for significant reductions in electric energy usage. These savings can be achieved while maintaining or enhancing indoor air quality through filtration of incoming and recirculated air in building HVAC systems. This paper reviews the technologies behind air filtration in buildings, and describes methods to select air filters, measurements of electric energy usage from fans and blowers, as well as field performance of particulate air filters. Using the criteria described, air filters and energy usage are evaluated in several common settings, including office and mixed use buildings, manufacturing plants, laboratories and hospitals. Several case studies are briefly described, showing in-situ filtration performance, electric energy measurements, life cycle cost calculations and total cost of ownership for air filters of various designs and qualities. Air filter performance is characterized using the test procedure outlined in ASHRAE Standard 52.2-2007 App. J, which includes a discharge step that neutralizes the temporary static charge present on some types of filter media. This step is important because the temporary static charge can result in inflated air filter "MERV (minimum efficiency reporting value) ratings that are higher than those exhibited by affected filters when deployed in air handlers shortly after installation in an AHU. Air filter guidelines and Ventilation for Acceptable Indoor Air Quality in occupied buildings is discussed in ASHRAE Standard 62.1-2007. In-situ air filter performance is measured using the detailed procedures outlined in ASHRAE Guideline 26-2008. These procedures include field measurements of ambient air and downstream particle concentrations, air flow across the filter bank, and resistance to airflow from clean and loaded air filters. "Cost of ownership" factors include air filter purchase price, operating cost (fan energy), installation labor and disposal. Electric energy measurements are made using a power data logger and other equipment used to evaluate motors and drives. Fan energy and air flow interactions with other system components are discussed, including constant and variable air volume system designs and variable frequency drives.