This article presents the Economic Security Index (ESI), a new measure of economic insecurity. The ESI assesses the individual-level occurrence of substantial year-to-year declines in available household resources, accounting for fluctuations not only in income but also in out-of-pocket medical expenses. It also assesses whether those experiencing such declines have sufficient liquid financial wealth to buffer against these shocks. We find that insecurity—the share of individuals experiencing substantial resource declines without adequate financial buffers—has risen steadily since the mid-1980s for virtually all subgroups of Americans, albeit with cyclical fluctuation. At the same time, we find that there is substantial disparity in the degree to which different subgroups are exposed to economic risk. As the ESI derives from a data-independent conceptual foundation, it can be measured using different panel datasets. We find that the degree and disparity by which insecurity has risen is robust across the best available sources.