In some ways ideas on fusion reactors have notchanged during the past twenty years—and in otherways the total development of conceptual reactordesigns has occurred within that time. Now, as then,most practical reactor designs are based on the reactionbetween deuterium and tritium at temperatures inexcess of 108 degrees, requiring a breeding blanketcontaining lithium to regenerate the tritium fuel andsubstantial shielding to reduce radiation levels beyondthe reactor. On the other hand, the form and structureof a reactor was only vaguely perceived twenty yearsago, whereas conceptual designs have now beendeveloped in great detail and described at great length.The associated technology has also developed rapidlyin recent years, and is now an essential and significantpart of any major fusion programme.