During the past 25 years, considerable progress has been made in the knowledge and understanding of engineered geothermal reservoirs, i.e., Hot Dry Rock and Hot Wet Rock reservoirs. Initial experiments at the Fenton Hill site in the USA have shown that it is possible to drill into hot, impermeable granite and create flow passages that stay open despite the high earth stresses tending to close fractures. Experiments in reservoirs in England, France, and Japan show that it is possible to produce thermal power at rates up to 10 MW with low resistance to flow through the reservoirs and acceptable ratios of production-to-injection rates. It still must be established that such reservoirs can be commercially successful. To do so, reservoirs must be scaled up in size, costs must be reduced, and improved methods should be developed for more accurately assessing the reservoir size and ultimate heat extraction capacity from microseismic and other geophysical and geological data.