Changes in lifestyles have gradually led to shifts in individuals’ physical activity habits, which can have significant consequences for their health. Opportunities for physical activity have diminished, largely due to the increasing use of motorized transportation and electric household appliances, the decline in physically demanding manual tasks at work, and the emergence of new sedentary leisure activities. It is now well established that insufficient physical activity, combined with an unbalanced diet, is a major determinant of major chronic diseases such as cardiovascular disease, type 2 diabetes, obesity, high blood pressure, and certain types of cancer. Globally, the World Health Organization (WHO) estimates that physical inactivity causes approximately two million deaths worldwide each year and is responsible for 10 to 16% of cases of breast and colorectal cancer and diabetes, as well as approximately 22% of cases of ischemic heart disease. [excerpt from chapter introduction]