In this work, we present a method for the online estimation of particle size distributions in layering granulation processes using a methodology known as model-based measurement or state estimation. After presenting the necessary model equations for two practically relevant processes it is investigated which quantities of the processes have to be measured to estimate the size distribution. For the different processes square-root unscented Kalman filters (SR-UKF) are designed and tested in simulations to demonstrate the feasibility of this approach. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 929-941, 2011