Since the discovery and maturing of in vitro techniques that characterized neurotransmitter receptor systems in animal and human brain and subsequent development of in vitro and in vivo autoradiography, there was always a clear scientific motivation to carry out these measurements in living brain. In the early 1980s, this was realized by both in vivo preclinical and ultimately human imaging of neurotransmitter (NT) systems starting with dopamine opiate and serotonin system. Now, some 30 years later, the in vivo imaging of (NT) receptor systems, primarily employing the methodology known as positron emission tomography (PET), single photon imaging computer tomography (SPECT) and other chemical measures including magnetic resonance spectroscopy (MRS) and pfMRI (pharmacologic magnetic resonance imaging) have dramatically established these approaches as valuable tools for neuroscience research.