System parameters as spatial resolution, sensitivity, singles and coincidence count rates, signal to noise (S/N) ratio have been measured with positron emitting radiopharmaceuticals in a Siemens MultiSpect2 system with 511 keV collimators, a Siemens ZLC dual-headed uncollimated rotating gamma camera system operated in coincidence mode and a Siemens Ecat 951/31 PET ring system. From these measurements it is evident that the sensitivity and spatial resolution of PET systems are superior to those achieved by SPECT systems. The performance of PET ring systems is superior compared to dual-headed coincidence systems especially if count rate capability and accidental coincidences are considered. The better energy resolution of NaI in comparison with EGO results in a lower scatter fraction at the same energy window. An absolute quantitation is only possible with PET ring systems with an individual attenuation correction.