Nuclear Medicine involves studying the time course of radioactive tracers and the physiological response of the body in vivo using external imaging devices. These devices are quantitative and can be used to assay tracer concentrations. This chapter briefly discusses the historical milieu, current state-of-the-art in terms of both single photon and positron tomography (SPECT and PET), and shows a semi-quantitative example of monitoring response to anti-cancer treatment with readily available instrumentation (a gamma camera) and radiopharmaceutical ([(18)F]-FDG). Nuclear Medicine is being increasingly utilised in drug discovery and development, and its role looks set to increase even further in the future.