The review describes the three most commonly used methods of separating 99mTc from 99Mo, namely chromatography, sublimation and solvent extraction. General comparisons are made between the various generator systems and their respective advantages and weaknesses. The method of producing the parent radionuclide 99Mo often dictates which of the generator options is more appropriate to a particular 99mTc user. Although the use of fission-produced 99Mo is widespread, this technique is not ideal since it requires considerable capital investment and gives rise to large quantities of long-lived radioactive waste. In certain countries such resources cannot be presumed and as a result alternative methods of producing 99mTc from neutron-activation-produced 99Mo are attractive. Recent advances in generator technology indicate that neutron-activation-produced 99Mo may eventually replace the need for fission-produced 99Mo. The review mentions one method of achieving this goal.