This chapter provides a comprehensive review of body composition, its importance for health, and strategies for its optimization. Body composition—the proportions of fat, muscle, and bone in the body—is a key determinant of overall health and fitness. Deviations from a healthy body composition, such as excess body fat or low muscle mass, are associated with various health risks. The chapter discusses different methods for measuring body composition, each with its unique advantages and disadvantages, ranging from simple techniques like bioelectrical impedance analysis to more complex methods like MRI/CT and densitometry. The implications of body composition on physical and mental health are explored, highlighting its impact on metabolic function, cardiovascular health, immune response, physical performance, and mental well-being. The chapter also delves into the changes in body composition associated with metabolic and bariatric surgery (MBS) and their implications for patient outcomes. This chapter underscores the multifaceted nature of body composition optimization, which involves a combination of dietary interventions, physical activity, lifestyle modifications, and potentially medical or surgical interventions. Tailored strategies are needed for specific populations, including older adults, athletes, children, adolescents, and pregnant/postpartum women. The chapter concludes with key take-home messages, emphasizing the need for healthcare professionals to incorporate body composition assessment and management into clinical practice.