This chapter seeks to familiarize the readers with the concept of diet and nutrition, its impact on individual’s health, alterations in dietary needs across the lifecycle and differences in needs as per activity levels and health condition, role of nutrition in management of good health, fitness and chronic degenerative diseases. Components of energy expenditure include basal metabolic rate (BMR), thermogenic effect of food, and physical activity. Energy intake and energy expenditure both determine the energy balance, body composition, health and risk for diseases in an individual. With respect to the variation in age, life cycle, body composition and physiological conditions there can be variations in the nutrient requirements of individuals. Especially in vulnerable age groups, i.e. pregnancy, lactation, childhood, adolescence, and elderly. Assessment of nutritional status of individuals, nutrition for exercise, and nutrition in lifestyle diseases are also discussed briefly in this chapter.