Flocculation processes are of great importance for numerous industrial processes, such as paper manufacture, mining, and the treatment of industrial and municipal waste waters. Polymeric flocculants having molecular weights up to 1 · 107 g/mol are generally used for this purpose. The flocculation mechanism (bridging, charge-mosaic) is determined by the chain length, the charge density and the molecular weight of the polymeric flocculant. There is a closely defined optimum for the dosing rates of flocculants, from both an ecological and an economic point of view. The specific choice of flocculants therefore requires a reliable measuring method with which tests of effectiveness can be carried out, ideally under service conditions. This paper describes a fiber-optic flocculation sensor (FOFS) for measuring the flocculation state in flowing systems, which has proved useful in practice for both laboratory flocculation tests and as a measuring component of control systems for dosing equipment. The focus of this contribution is the use of the FOFS for assessing the effectiveness of polymeric flocculants in relation to waste water treatment.