Radioligand studies enable visualisation and measurement of molecular pathways and pharmacokinetic processes. Using positron emission tomography, accurate measurements of the time course of radioligand uptake and clearance can be obtained. A tracer kinetic model is needed to derive physiological or pharmacokinetic parameters from these tissue time-activity curves. In addition, an input function that indicates delivery to the tissue is required. Usually this will be the arterial plasma curve. For receptor studies, where binding potential is the parameter of interest, it might also be possible to avoid arterial sampling provided a tissue can be defined that is devoid of receptors.