Helium is not a rare gas if we can afford to fill the world’s balloons with it. Actually, balloons account for only 16% of helium use. Cooling of semiconductors accounts for 33%, and the rest is used for industrial and scientific purposes. The atmosphere contains four billion tonnes of He, but it is not economical to extract it by cryo — distillation. Most of our helium comes from natural gas as a by-product. Thus, helium comes from fossil fuels and will be depleted in several decades along with natural gas, as discussed in Chap. 2. In this chapter, we have seen how critically fusion reactors, as envisioned today, will depend on helium in both extremely hot and extremely cold places. In the first wall and blankets, gaseous helium is used as a high-temperature coolant. The vacuum system uses liquid helium to cool the cryo — pumps. In the magnet system, liquid helium is what produces superconductivity. It is a closed system, but there are leaks. It is estimated that ITER will lose 48 tonnes of helium a year, about 0.15% of the world’s current consumption. But if eventually fusion produces a third of the world’s power, those reactors would need the world’s supply of helium for a whole year just to start up [4]. At some point the helium losses, say, 10% of the inventory, would exceed what comes from natural gas. You will remember that helium is one of the products of the D-T reaction. At only a few percent burnup, however, this “ash” is a negligible contribution to the total demand. Helium is needed in other industries as well; for instance, in medical equipment. The shortage is so acute that a rationing system was proposed in the USA in 2010.