Positron emission tomography (PET) is a quantitative in vivo tracer technique, enabling images of the distribution of biochemical, physiological and pharmacological functions in living tissue, at a resolution of a few millimetres. Applications include the imaging of blood flow rate, metabolic rate and neuroreceptor distribution and function. These applications are playing an increasing role in drug development. This brief article seeks to emphasize how these applications of PET need to rest on a solid quantitative foundation.