This is an exciting time for PET technology where we anticipate major developments in instrumentation that might lead to breakthoughs in clinical and research applications. Over the years, different design trends have emerged, with PET scanners now available with a broad spectrum of features, from those available commercially for clinical applications to others designed primarily in research laboratories specifically for very high-resolution research applications. The latter category includes organ-specific (brain, breast, prostate) and small-animal imaging systems.