We study household food security dynamics in the United States from 2001 to 2017 using 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 and subpopulation-scale dynamics by investigating the conditional distribution of estimated food insecurity spells and the chronic and transient components of estimated food insecurity. We find that two-thirds ofhouseholds experienced no estimated food insecurity during the 2001-17 period and more than half of newly food insecure households regain food security within two years. Households headed by female, non-White, or less educated individuals disproportionately suffer persistent, chronic and/or severe food insecurity.