Laboratory studies of the Forward Scattering Spectrometer Probe (FSSP), were conducted to better understand the operation, to determine limitations and to define the measurement accuracy of the instrument for airborne cloud physics research. The studies included electronic cheeks of the instrument sensitivity to simulated particles of different sizes, airspeeds and arrival rates; measurement of important aspects of the optical configuration; and intercomparisons of six different FSSPs in a small wind tunnel with a droplet spray. The tests demonstrated measurement differences between various probes in several areas as well as areas in which there was reasonable agreement. Part of the differences can be attributed to different feature and design specifications of different probes as changes were made to improve the FSSP. Areas in which care needs to be taken in the calibration and processing of data from the FSSP are identified.