One of the major product specifications of a crystalline material is the crystal size distribution (CSD). In order to monitor and control the CSD in an industrial crystallization process, on-line sensors are required. Over the years, a number of techniques to measure the CSD have been established. In this paper, three instruments operated in an on-line fashion and an off-line method are compared. The instruments were the OPUS, a HELOS/VARIO (both manufactured by Sympatec) and a Malvern 2600c (manufactured by Malvern). They were implemented on an 1100-1 evaporative draft tube baffle (DTB) crystallizer producing ammonium sulfate crystals from aqueous solution. Samples from this crystallizer were also analyzed offline by wet sieving. The results show reasonably good agreement between the different on-line techniques and the wet sieving technique concerning the shape of the distribution. However, there is a discrepancy regarding the absolute values, which can be explained by the fact that the techniques used are based on different measuring principles.