This lecture delves into the foundations of group behavior, explaining why individuals join groups and how group dynamics evolve. It introduces key theories: Propinquity Theory, which links group formation to physical proximity; Exchange Theory, based on cost-reward interactions; Balance Theory, emphasizing shared goals and attitudes; and Homan’s Theory, which ties group bonding to shared activities and interactions. The lecture also explores major models of group development, including Tuckman’s five-stage model, Tubbs’ systems approach, and Gersick’s Punctuated-Equilibrium Model, offering valuable insights into how groups form, function, and adapt over time.