Since positron emission tomography (PET) and single photon emission tomography (SPET) have become widely available as imaging modalities, they have been used to perform studies on neuroreceptor physiology and pathophysiology in man and animals. Several neurological and psychiatric diseases related to neurotransmitters and receptor functioning have been investigated. In order to investigate the biological effects of new compounds, this imaging modality is becoming more and more popular in the pharmaceutical industry, since pharmacokinetics and pharmacodynamics of new drugs can now be evaluated in vivo, omitting extensive in vitro examinations. The response to pharmaceuticals can thus be evaluated more objectively than with the more subjective assessment of patient condition based on clinical examination and the interpretation of neuropsychological tests. This review focuses on the technology of this imaging modality and on the nature of the radioligands used for neuroreceptor imaging. A brief overview will be given of the research on neuroreceptor physiology, neuropathology, drug evaluation and substance abuse, with the emphasis on animal models. Future prospects for implementation in veterinary medicine are discussed.