The determination of the properties of receptors has been made possible by the development of high specific activity radiotracers. There are several classes of biochemical radiotracers that can be studied. Of these classes, receptor binding radiotracers are especially interesting in that changes in receptor concentration are thought to be related to certain disease states. Using a radioligand deemed to be sensitive to receptor change, clinical studies must be carried out verifying their clinical sensitivity and selectivity. This is the challenge for single-photon and positron emitting, receptor-binding radiotracers in the coming years. The development of a flow tracer that is insensitive to biochemical changes, especially in disease states, has not been validated to date for single-photon emitting radiotracers. The major focus of gamma-emitting receptor binding radiotracers has definitely been on the steroid hormones. Receptor binding radiotracers offer an area of research that will allow the noninvasive monitoring of the change in receptor as a function of disease.