Thermonuclear reactions under steady-state conditions are considered in order-of-magnitude terms. Energy loss by radiation and the transfer of energy between nuclei and electrons are also discussed. It is pointed out that the principal problem is constructing a suitable “magnetic bottle” in which nuclei of a dilute, completely ionized gas (e.g., H2, H3) at a temperature 108 °K can be confined and reacted before losing too much energy to the walls. The practical confinement of the plasma, involving substantial hydromagnetic difficulties, can probably be accomplished, although it appears to be perhaps decades in the future. Potential advantages of a thermonuclear reactor over a fission reactor include: virtually inexhaustible fuel supply available, fuel reprocessing unnecessary, no chain reaction run-away hazard present, and direct conversion of thermonuclear energy to electrical energy may be possible.