Food security in the U.S. has been assessed using a discrete measure over short periods, providing little information on its long-term persistence and severity, nor on the intensive and potentially heterogeneous effects of a safety net program designed to improve food security. This dissertation consists of three chapters to shed light on this missing knowledge. Chapter 1 introduces the motivation for and overview of this research. Chapter 2 investigates food security dynamics and severity over a 17-year period (2001-2017), using a new food security measure called the Probability of Food Security (PFS). I show that most food insecurity experiences are transitory, but there exist substantial gaps in food insecurity and persistence across subgroups. Chapter 3 examines food security dynamics over a longer period (1979-2019) and a more inclusive sample using the same measure. The study finds that food insecurity has been largely associated with sex, racial and educational characteristics, and that more than half of food insecurity spells are transitory, lasting two years or less. Chapter 4 evaluates the effects of the Supplemental Nutrition Assistance Program (SNAP) on food security using state-level SNAP administrative policies as a source of exogenous variation. I find that state policies have significant effects on SNAP participation, but SNAP does not have significant effects on improving food security on average.