Significance In the past decade, sporadic shortages of metals and metalloids crucial to modern technology have inspired attempts to determine the relative “criticality” of various materials as a guide to materials scientists and product designers. The variety of methodologies that have been used for this purpose have (predictably) resulted in widely varying results, which are therefore of little use. In the present study, we develop a comprehensive, flexible, and transparent approach that we apply to 62 metals and metalloids. We find that the metals of most concern tend to be those with three characteristics: they are available largely or entirely as byproducts, they are used in small quantities for highly specialized applications, and they possess no effective substitutes.