Nowadays, a growing number of research groups shows a great interest for the application of PET and SPECT techniques to the development of new drugs. Preliminary studies on small animals require high performance dedicated scanners with a higher spatial resolution and sensitivity than those of clinical systems. In this paper the potential applications of such innovative instruments are shown together with a brief review of the dedicated PET and SPECT tomographs developed worldwide. Most of the scanners have been built as research prototypes. Only two are commercially available: micro-PET(R), designed and developed at UCLA, Los Angeles as a research prototype, and now produced and distributed by Concorde Microsystems Inc. (USA) and HIDAC PET produced by Oxford Positron Systems Ltd. (UK). Also in Italy, a high performance tomograph, YAP-(S) PET able to perform both PET and SPECT studies, has been developed at the University of Ferrara. The technical characteristics and performance of this scanner are described. Tomographs with combined imaging techniques, such as PET/CT or SPECT/CT, are now under study in various international research centers. The advantages of this new generation of animal scanners will be briefly outlined.