Underregulation is a common and persistent environmental law problem, with recent scholarly focus on individual contaminants of emerging concern (CECs), whose harm is not fully known. But little attention has been given to the general trend of underregulation with respect to these chemicals, or explaining why this systematic underregulation occurs. This Article posits that federal agencies have been unacceptably slow to initiate protective regulations, and even once regulations are promulgated, they leave regulatory gaps that continue to expose populations to harmful effects. It further argues that the scientific uncertainty that defines CECs as a class is responsible for this particularly significant pattern of underregulation, and obscures the existence and scope of the problem from the public and from regulators themselves. The Article concludes with recommendations to address these shortfalls.