Poverty-related food insecurity is a reality that many clinicians in nutrition and health care encounter either directly or indirectly. It is associated with both overnutrition and undernutrition, but it is not congruent with malnutrition. Food insecurity affects human development and health throughout the lifecycle, but can be particularly harmful during critical or vulnerable stages early and late in life. Understanding the causes and consequences of food insecurity and knowing how to identify them can improve the quality and effectiveness of clinical care, and facilitate prevention and treatment of many kinds of health problems.