Surface plasmon resonance (SPR) biosensors are optical sensors exploiting special electromagnetic waves-surface plasmon-polaritons-to probe interactions between an analyte in solution and a biomolecular recognition element immobilized on the SPR sensor surface. Major application areas include detection of biological analytes and analysis of biomolecular interactions where SPR biosensors provide benefits of label-free real-time analytical technology. This paper reviews fundamentals of SPR affinity biosensors and discusses recent advances in development and applications of SPR biosensors.