Food insecurity is a leading public-health challenge in the United States today. This is primarily due to the magnitude of the problem—about 50 million persons are food insecure—and the serious negative health and other outcomes associated with being food insecure. This chapter first defines the measure used to delineate whether a household is food insecure. The measure, the Core Food Security Module (CFSM), is based on 18 questions about a household’s food situation. From the responses, a household is defined as food secure, low food secure, or very low food secure, with the latter two categories defined as “food insecure.” I next discuss the extent of food insecurity in the United States across various dimensions, the key determinants of food insecurity, and the multiple negative consequences associated with food insecurity. Two of the key policy tools used to address food insecurity are the Supplemental Nutrition Assistance Program (formerly known as the Food Stamp Program) and the National School Lunch Program. A brief overview and definition of the eligibility criteria for each program is provided along with a discussion of their respective impacts on food insecurity. This chapter concludes with four major current challenges pertaining to food insecurity and food assistance programs.