This paper studies household food security dynamics in the United States from 2001 to 2017. We introduce a new measure, the probability of food security (PFS), the estimated probability that a household’s food expenditures equal or exceed the minimum cost of a healthful diet. We use PFS to analyze household-level as well as subpopulation-scale dynamics by investigating both the conditional distribution of food insecurity spells and the chronic and transient components of food insecurity over an extended period. More than half of newly food insecure households resume food security within two years. Households headed by female, non-White, or less educated individuals disproportionately suffer persistent, chronic food insecurity.