Surface Plasmon Resonance (SPR) is an optical technique that uses evanescent waves as a valuable tool to investigate chemical and biological interactions taking place at the surface of a thin sensing layer. SPR offers a real time analysis of dynamic adsorption and desorption events for a wide range of surface interactions. After a brief theoretical introduction, examples of a wide range of applications of SPR are presented. Main application areas involve the detection of biological analytes and study of biomolecular interactions in liquid phase. Applications in chemical sensors will be also illustrated by using different classes of organic and inorganic material as sensing layers.