This chapter discusses the effects of physical activity on individual health, the prevention of chronic diseases, and overall quality of life. A sedentary lifestyle is one of the main causes of many diseases, including obesity, type 2 diabetes, coronary heart diseases, certain types of cancer, musculoskeletal disorders, and mental health problems. Regular physical activity plays a crucial role in controlling body weight, lowering blood pressure, regulating glucose metabolism, supporting bone health, and reducing stress and depression. It also contributes to the development of social relationships, enhances quality of life, and helps reduce healthcare costs. Within this context, both subjective and objective assessment methods are used to determine physical activity levels. The intensity of activity is categorized as low, moderate, or high, while the individual's energy expenditure is measured according to their physical activity level. This chapter explains in detail the contributions of physical activity to public health, its economic benefits, and the negative consequences of physical inactivity.