Résumé : Although going to complex techniques can be useful at some stage of process development of crystallization processes, the goal of this article is to show that process control can also be achieved with relatively simple measurements, depending of course on what type of control is needed. This work demonstrates the potential of the online monitoring of the power dissipated in the suspension, a property which can be directly related to the suspension apparent viscosity for the understanding and the control of crystallization processes. Focusing on the Etiracetam crystallization process, it is shown that the online monitoring of the suspension apparent viscosity allows to pinpoint the moment of the polymorphic transformation from form II to form I of Etiracetam. Moreover, it allows identifying it as a solvent-mediated polymorphic transformation. The monitoring of the apparent viscosity of a suspension is an easy-to-use, friendly and less expensive tool compared to other sophisticated PAT tools.