The critical metals are vital to modern life due to their use in a variety of domestic, green, and military high technology applications but have supplies that are inherently insecure. This study provides an overview of the concept of criticality as applied to the critical metals and outlines key issues around the resources and future supply of these metals. The methods used to quantify the criticality of critical metals have advanced over time, demonstrating that some metals are more strategically important than others, depending on the viewpoint of the organization considering criticality. However, global resources and reserves of a number of critical metals as well as their production statistics remain unclear. Methods exist to quantify the resources of critical metals with reasonable accuracy but these methods rely on information provided by the mining industry, indicating that better reporting practices would improve our knowledge of the global resources and cycling of these key commodities. Criticality can also be addressed in numerous ways, including the analysis of known mine supply chains to enable the economic extraction of critical metal by-products, the determination of the critical metal prospectivity of mining/mineral processing wastes (given a significant amount of critical metals currently deport to waste), increased amounts of recycling intermediates or end-use products containing critical metals, and the discovery of new and economic deposits of the critical metals. However, all of these approaches and the associated policy around them require more information in terms of mineral resource accounting, mineral economics, material flow analysis, mineral processing, as well as increased economic geology knowledge that would enable the making of future discoveries and increase the likelihood of critical metals being extracted as either primary or by-products. Without this information, significant parts of our knowledge base on the supply (and the security of this supply) of the critical metals will remain opaque.