Biosensors are heterogenous devices, incorporating biological struc- tures combined with electronics, optical or other readout systems. They have been developed for detecting different biomolecules and/or pathogens and represent a key technology for advanced and point- of-care diagnostics as well as patient monitoring. In this paper we present a systematic classification of biosensors described in litera- ture, particularly focusing on nanotechnology-based sensing. Then, we present our approach to develop electrochemical biosensors for measuring metabolites and anticancer drugs, based on a platform for multiple target detection. This platform is modular and achieves a clear separation between the chemical and the electrical compo- nents, thus easing design and manufacturing. It shows superior per- formance thanks to the excellent properties of electron transfer and selectivity showed by enzymes immobilized on carbon nanotubes.