setup of an SPR experiment is simple: There is a sensor surface to which one of the interacting partners (the ligand) is immobilized; the other partner (the analyte) is added in a ?ow or cell-like compartment. The binding phenomenon is monitored in real time as a change in SPR angle. An important issue is the choice of surface and the immobilization strategy. With SPR, it is possible to mimic the biological environment which is relevant for an interaction. For interactions in a water environment, sensor surfaces with hydrogels are available. Many biomolecular interactions take place in a membrane environment. For this, commercial sensor surfaces are available, or surfaces can be tailor-made. This volume contains several examples of building up of lipophilic surfaces. Nature abundantly makes use of multivalent interactions; multivalency can be mimicked on a sensor surface with immobilized ligands.