In this chapter, we describe the essential program ingredient of belonging and why it is foundational in program settings to ensure that youth thrive. The concept of belonging is not new and can be traced back to early psychological theorists such as Freud, Bowlby, and Maslow. Until recently, belonging has not been clearly defined conceptually, but at its core, it refers to the concept of fitting in, being respected for who you are, and mattering in a group. We review how belonging has been defined, how it has become important for positive youth development program settings, how it interacts with different factors, how it is important to the youth development workforce, and how belonging is associated with youth well-being. We also summarize key takeaways for how youth development staff can foster a sense of belonging in programs.