Aerosol mass spectrometers measure the time-resolved chemical composition of airborne particles. Our laboratory has developed several instruments, each targeting a different combination of particle size and chemical components. These instruments have been used in the field to characterize ambient air and in the laboratory to study chemical processes associated with particle formation and growth. This presentation will provide an overview of the methodology and applications of aerosol mass spectrometry. Special emphasis will be given to ambient measurements in the mid-Atlantic region.