Biosensors are heterogenous devices, incorporating biological structures 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 literature, 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 components, thus easing design and manufacturing. It shows superior performance thanks to the excellent properties of electron transfer and selectivity showed by enzymes immobilized on carbon nanotubes.