Energy-critical elements are a class of chemical elements that currently appear critical to one or more new energy-related technologies. A shortage of these elements could significantly inhibit large-scale deployment, which would otherwise be capable of transforming the way we produce, transmit, store, or conserve energy. The 2011 report titled “Energy Critical Elements: Securing Materials for Emerging Technologies” by the American Physical Society’s Panel on Public Affairs and the Materials Research Society surveys potential constraints on the availability of these elements. The report addresses elements that have not been widely extracted, traded, or utilized in the past, and are therefore not the focus of well-established and relatively stable markets. The report discusses a number of constraints on the availability of ECEs for the U.S. and world markets: (1) Crustal abundance, concentration, and distribution. Whereas exploration benefits from well-tested geological models of ore deposits for the more common metals, such understanding is lacking for many of the less common elements. (2) Geopolitical risk. The production of some ECEs is dominated by one or a few countries. (3) Risk of joint production. Tellurium and selenium are good examples of ECEs that are produced as byproducts of a more common metal – copper. There is little incentive to increase the production of these byproduct metals, as long as their prices remain low relative to their abundances. (4) Environmental and social concerns. As countries that now have lax environmental, safety, health, and social impact standards embrace higher standards, the price and availability of ECEs may be significantly affected. (5) Response times in production and utilization. The time period from exploration to production is commonly 5 to 15 years or longer, and there are similarly long timeframes, sometimes decades, for bringing a new technology, such as a new choice of elements for photovoltaics, to market.