Radioisotope thermoelectric generators (RTG) convert the decay energy of a radioisotope ( 238 Pu) into heat then into electricity. RTGs have been used to power space exploration missions. This review paper studies several crucial features of past and present Static RTGs, for instance the advantages of Static RTGs. In addition it reviews Static RTGs limitations such as the shortage in the amount of 238 Pu in (U.S.A). Furthermore it compares the future Dynamic RTGs with Static RTGs. It indicates the future Dynamic RTGs, which include a thermodynamic cycle, use 2-4 times less amount of 238 Pu. In spite of that, their efficiency is almost 4 times greater than past and present RTGs.