Some considerations are given that should be regarded when selecting a sensor for the control of the particle size distribution (PSD) in particulate processes, such as crystallizers, yet-mills and granulators. As an example, the control of the crystal size distribution (CSD) dynamics in a continuous crystallization process is taken. Different methods are discussed to derive process information from a typical PSD sensor, comprising an array detector. It is shown that generally a limited number of degrees of freedom are present in the process output signals. This is due to the principle of measurement and the limited number of degrees of freedom in the process itself. Factors limiting the controllability of processes are discussed. It is shown that proper placement of the sensor in the particle size domain is important for feedback control, because the sensor characteristics will determine the location of the zeros in the process transfer function. For an industrial crystallization process, it is shown that measurement of the CSD below 30 mu m limits the achievable closed-loop speed of response, due to the presence of a right halve plane zero. Effective control of the CSD is achieved, using a simple feedback controller, that measures only the fine crystals in the range of 40-200 mu m. Experimental results from the UNIAK pilot crystallizer are depicted.