In the recent times, the focus on attaining nutrition security has increased in both developed and developing countries. An important step in solving issues related to nutrition is to define and measure it. This lecture presents the basic concepts of nutrition security and explains different ways of measuring it. Building on existing literature and the extensive work done by IFPRI over the past four decades in this area, this lecture is for students, policy makers, and other relevant stakeholders interested in gaining a deeper understanding of nutrition security.