In the fields of analytical and physical chemistry, medical diagnostics and biotechnology there is an increasing demand of highly selective and sensitive analytical techniques which, optimally, allow real- time label-free monitoring with easy to use, reliable, miniaturized and low cost devices. Biosensors meet many of the above features and, consequently, have gained a place in the analytical bench top as alternative or complementary methods for routine classical analysis. Different sensing technologies are being used for biosensors and, among them, piezoelectric micro-machined devices (pMMD) represent a cost-effective approach that have the potential to identify with high sensitivity various species in the gas phase as well as in the liquid phase. Moreover, pMMD can be designed to allow portability, integration capabilities, low power consumption and easy employment. Piezoelectric sensors are often collectively referred to as mass-sensitive devices since they are able to detect mass change on their surfaces. For biosensor applications, the surface of the piezoelectric device is modified with recognition elements (e.g. antibodies) that, binding specifically the target analyte, give rise to changes of the mass. pMMD are based on the propagation of acoustic waves and they are classified according to the characteristic of the generated waves (i.e. direction of particle displacement, wave-guiding mechanism, etc.). The most used devices, suitable for different applications, are: Rayleigh surface acoustic wave (SAW) devices, shear horizontal (SH) SAW devices, thin film bulk acoustic resonators (TFBAR), lateral field excited bulk acoustic resonators (LFEBAR), contour mode resonators (CMR) and Lamb wave devices.