Recent developments in the use of pinhole SPECT and dedicated PET for UHR small animal imaging have identified the technology that can be used to provide images with spatial resolution of the order of 1 to 3 mm. In SPECT imaging, rotating camera pinhole SPECT has provided the means to use existing equipment to achieve UHR images. It has the disadvantages of low sensitivity and requires special image software to reconstruct tomographic slices. However, with minimal additional cost to an imaging laboratory, pinhole SPECT can provide a quantitatively accurate imaging technique for small-animal studies. New detector technology offers considerable promise; however, more studies are required before any one system can be singled out as offering major advantages over the pinhole SPECT method for general purpose small-animal SPECT imaging. The search for the means to achieve better sensitivity with UHR continues. In PET imaging, with few exceptions, the general trend has been to design systems dedicated to small-animal imaging to achieve UHR images with satisfactory sensitivity for quantitative UHR imaging. Several of the ring configuration, small-animal PET imaging systems provide good sensitivity and high spatial resolution. The cost of many of these systems, however, is relatively high, and investigators continue to explore different detector materials and imaging geometries to develop an instrument with acceptable levels of sensitivity with UHR imaging capability. We believe that both small-animal SPECT and PET imaging techniques now offer practical UHR imaging methods for quantitative small-animal imaging. As these tools are implemented in the investigation of new radiopharmaceuticals, we expect the utility of in vivo small animal assays will support further research in optimizing this technology.