Time-of-flight (TOF) aerosol analyzers are a class of instruments that measure the aerodynamic diameter of individual particles following a controlled acceleration in a well-defined flow field. Two instruments have been used to analyze the size of medical aerosols: Aerosizer particle size analyzer (TSI Particle Instruments/Amherst, Amherst, MA), Aerodynamic Particle Sizer (APS) aerosol spectrometer (TSI) Both instruments are capable of sizing several thousand particles a second, making it possible to obtain aerodynamic particle size distributions in a few seconds compared with up to 1 hour per measurement using compendial methods that are based on either the multistage liquid impinger or cascade impactor. This rapidity makes TOF analysis attractive for product development, as many different variables can potentially be investigated during a short period of time. The data thus obtained should be used with caution, however. Several issues, most notably the lack of a direct relationship with the mass of drug substance present and the vulnerability of the measurements to coincidence effects when sampling concentrated aerosols, may severely limit the value of data from many aerosol delivery systems, especially pressurized metered dose inhalers (pMDIs). A review of the literature illustrating the issues that are involved and providing guidance on the most appropriate uses of these analyzers is presented.