The physical properties of nuclear fusion reactions govern the fuel choice and the selection of the operating plasma temperature. Scientific feasibility, or energy break-even, is one objective of the current research program. Engineering and economic feasibilities will depend on achievable plasma densities at thermonuclear temperatures and on technological questions of achievable magnetic field, effects of radiation damage, and mechanical design problems. A pure fusion reactor will have stringent requirements on its performance parameters. If achieved, it should provide clean, safe, abundant economic power, but from rather large central stations. Hybrid fission/fusion systems put far less stringent requirements on the plasma performance. They simplify the fission lattice design. They may provide a superior method for burning the vast reserves of uranium-238.