A review of droplet sizing instruments used for icing research is presented. These instruments include the Forward Scattering Spectrometer Probe, the Optical Array Probe, the Phase Doppler Particle Analyzer, the Malvern Particule Size Analyzer, the oil slid technique, and the rotating multicylinder. The report focuses on the theory of operation of these instruments and practical considerations when using them in icing facilities. The purpose of this SAE Aerospace Information Report (AIR) is to provide information that is useful for the selection, operation, and appreciation of the limitations of droplet sizing instruments when applied to aircraft icing research and development.