We show the design, fabrication and testing of micro/nanobiosensor devices based on highly sensitive optoelectronic and nanomechanical transducers. Most of the devices are fabricated by standard Silicon CMOS microelectronics technology according to a precise design for achieving a high sensitivity for biosensing applications. Three biosensors have been developed: (a) a Surface Plasmon Resonance biosensor, (b) an integrated, Mach–Zehnder micro/nano interferometer device based on optical waveguides, and (c) nanomechanical biosensors based on microcantilevers.