During the last two decades, functional brain imaging using positron emission tomography (PET) has advanced elegantly, and steadily gained importance in the clinical and research arenas. Significant progress has been made by different scanner manufacturers and research groups in the design of dedicated high-resolution three-dimensional (3-D) PET units; however, emerging clinical and research applications of functional brain imaging promise even greater levels of accuracy and precision and therefore, impose more constraints with respect to the intrinsic performance of the PET tomograph. Continuous efforts to integrate recent research findings for the design of different geometries and various detector/readout technologies of PET scanners have become the goal of both the academic community and nuclear medicine industry. As PET has become integrated into clinical practice, several design trends have developed; with systems now available with a spectrum of features, from those designed for "low cost" clinical applications to others designed specifically for very high-resolution research applications. There is also a continual upward revision and refinement in both hardware and software components for all of these systems. Software- and hardware-based correlation between anatomical (x-ray CT, MRI) and physiological (PET) information is a promising research field and now offers unique capabilities for the medical imaging community and biomedical researchers. One of the main advantages of dual-modality PET/CT imaging is that PET data are intrinsically aligned to anatomical information from the x-ray CT without the use of external markers or internal landmarks, thus providing a reliable estimate of the attenuation map to be used for attenuation and scatter correction purposes. On the other hand, combining PET with MRI technology is scientifically more challenging owing to the strong magnetic fields. Nevertheless, significant progress has been made resulting in the design of a prototype small animal PET scanner coupled to three multichannel photomultipliers via optical fibers, so that the PET detector can be operated within a conventional MR system. Thus, many different design paths have been and continue to be pursued in both academic and corporate settings, that offer different trade-offs in terms of their performance. It still is uncertain which designs will be incorporated into future clinical and research systems, but it is certain that technological advances will continue and will enable new quantitative capabilities in brain PET imaging. This paper briefly summarizes state of the art developments in dedicated brain PET instrumentation. Future prospects will also be discussed.