In activated sludge wastewater treatment research, most of the attention has been devoted to the biodegradation process. However, in terms of effluent quality, the final settling and clarification processes are at least as important because any particle carried over the overflow weir brings with it not only COD, but also nitrogen and phosphorus. In recent years we see indeed increased attention on this separation step, and alternatives such as membrane separation are increasingly introduced. Still, a large installed base of settlers exists, whose operation could/should be optimised. The measurement techniques presented in this contribution each focus on one of the key processes in the secondary settler: hydrodynamics, particle aggregation and breakage, hindered settling and compression. For each technique, the measuring principle is explained and a typical data set shown.