Optical biosensors, based on evanescent wave technology, are analytical devices that measure the interactions between biomolecules in real time, without the need for any labels. Specific ligands are immobilized to a sensor surface, and a solution of receptor or antibody is injected over the top. Binding is measured:by recording:changes in:the refractive index, caused by the molecules interacting near the sensor surface within the::evanescent field. Evanescent wave-based biosensors are being used to study an increasing number of applications in the life sciences, including the-binding and dissociation kinetics of antibodies and receptor ligand pairs, protein-DNA and DNA-DNA interactions, epitope mapping, phage display libraries, and whole cell- and virus-protein interactions. There are currently four commercially available evanescent wave biosensors on the-market :This article describes the:technology behind their sensing techniques,: as:well as the range of applications in which they are employed.