From their discovery, CNTs have increasingly attracted interest because of their peculiar electrical, mechanical, and chemical properties. In 1991, Sumio Iijima first observed and described in detail the atomic arrangement of this new type of carbon structure [1]. By a technique used for fullerene synthesis, he produced needle-like tubes at the cathode of an arc-discharge evaporator. From that time, carbon nanotubes have been used for many applications and represent one of the most typical building blocks used in nanotechnology. Their peculiarities include unique properties of field emission and electronic transport, higher mechanical strength with respect to other materials, and interesting chemical features. The use of CNTs has recently gained momentum in the development of electrochemical biosensors, since their utilization can create devices with enhanced sensitivity and detection limit capable of detecting compounds in concentrations comparable to those present in the human body. This chapter will review the most important features of carbon nanotubes, and present an example in which their application can enhance the detection of drugs and metabolites relevant in personalized medicine: P450 biosensors for therapeutic drug monitoring.