Hydrogels are cross-linked polymer networks able to absorb or to release large amounts of water. The water uptake is associated with a considerable volume change but also with changes of optical properties like the refractive index. The swelling can be excited by a large spectrum of different physical (e.g. temperature, electrical voltage, magnetic field) and chemical factors (e.g. pH value, concentrations of chemical or biochemical species). The particular sensitivity can be adjusted by tailoring the composition of the hydrogel or via its functionalization. If the interaction between hydrogel and analyte to be measured is reversible then such hydrogels are becoming a promising candidate for miniaturized, cost-effective and inline-capable sensors.