This chapter describes the application of nano- and micro-electromechanical systems (NEMs and MEMs), and more specifically microcantilever structures, as transducers for highly sensitive biosensors. In these devices, named as ‘nanomechanical biosensors,’ a biomolecular interaction produces a change in the mechanical behavior of the transducer (a movement at nanometer scale), which can be measured and analyzed in real time. Microcantilevers translate the molecular recognition of biomolecules into a nanomechanical motion that is commonly coupled to an optical read-out system. This chapter discusses the main aspects regarding the physics of microcantilever as well the optical read-out techniques. It reviews the state-of-the-art, and discusses the prospective future directions of this new family of biosensors. Nanomechanical sensors are derived from the microfabricated cantilevers used in atomic force microscopy (AFM) and are based on the bending or resonance change induced in the cantilever when a biomolecular interaction takes place on one of its surfaces. The cantilever response depends on its mechanical properties, which are determined mainly by their spring constant and resonance frequency. Both parameters depend on the cantilever material and its geometry. The increasing number of applications of microcantilevers as biosensors has established these systems as a versatile platform for real-time and in situmeasurements of physical, chemical, and biochemical interactions. Further research is banked upon to provide information for increasing the biosensor sensitivity.