The High Temperature Reactor commands a unique fuel cycle flexibility. This is demonstrated in the paper for the case of the commercial GA 1160 MW(e) station. The reactor is designed to operate on the thorium cycle with recycling of the bred U233. However, also the low-enriched uranium fuel cycle can be accommodated in the existing design without major alterations to the reactor and the shutdown system. The reference thorium reactor operating in the U233 recycle mode is 10–20% cheaper than the low-enriched reactor; however, the thorium cycle depends on the supply of 93% enriched uranium and the availability of reprocessing and refabrication facilities to utilise its bred fissile material. The closing of the thorium cycle is of paramount importance and a step to realise this objective lies in simplifying the head-end reprocessing technology by abandoning the segregation concept of feed and breed coated particles in the reference cycle. A one-coated-particle scheme in which all discharged uranium isotopes are recycled in mixed oxide particles is feasible and suffers a very minor economic penalty only. The economic landscape towards the end of the century will presumably be dominated by pronounced increases in the costs of natural resources. In the case of nuclear energy, resource considerations are reflected in the price of uranium, which is expected to have reached 50$/lb U 3O 8 in the early 1990's and even 100 $/lb U 3O 8 around 2010. In this economic environment the fuel cycle advantage of the thorium system amounts to some 30% and is capable of absorbing substantial expenses in bringing about the closing of the out-of-pile cycle. A most attractive aspect fo the HTR fuel cycle flexibility is for the utility to start operating the reactor on the low enriched uranium cycle and at a later date switch over to the thorium cycle as this becomes economically more and more attractive. As a result of detailed investigations this option is demonstrated for the GA 1160 MW design. It can be shown that the transition phase will cause no larger problems than the ones encountered in the normal operation of either of the cycles. The economic incentive amounts to some 50 Mio $ in terms of present worth money at the time of decision making assumed to take place 10 yr after start-up.