Noninvasive imaging of molecular events and interactions in living small animal models has gained increasing importance in preclinical research. Two of the imaging modalities available for this research with potential for translation to the clinic are dedicated small animal positron emission tomography and single-photon emission computed tomography. This brief review introduces the fundamental principles behind these imaging technologies and instrumentation, and discusses the limitations in terms of their spatial resolution and sensitivity. In addition, it provides a perspective regarding the research and commercial development of these systems and presents examples of biological applications. Finally, it discusses the major challenges facing these technologies, advantages and limitations with respect to other technologies, and some future prospects.