This contribution deals with the design of an observer for state estimation of a batch crystallizer, which is described by a detailed population balance model. Therefore, the rigorous model containing (partial) integro-differential equations is first reduced by applying an integral approximation technique to a model of moments that consists of only a few ordinary differential equations. This reduced model then serves as the basis for the design of a Luenberger type observer. The performance of the observer is finally demonstrated in various disturbance scenarios, using the rigorous population balance model for the simulation of the crystallizer plant. (c) 2007 Elsevier Ltd. All rights reserved.