In science, theoretical models are used to reduce the complexity of the portions of the world being studied in ways that make the subject matter suited for empirical investigation. Practitioners and researchers studying youth development have discovered that the biologically based “deficit” models of young people that were used in past decades to study young people were, at best, far too narrow and, at worst, wrong. At this writing, strength-based models of young people have proven more useful. In these models, youth are seen to possess developmental assets that can be promoted through strength-based youth development programs, such as 4-H. As illustrated in this chapter, it is highly likely that these models, including the 4-H Thriving Model (i.e., the theory of change for youth development in 4-H), will continue to evolve and thus continue to inform the practice of youth development.