Across the world, physical inactivity is responsible for five million deaths every year, and one in six deaths in the UK. This chapter explains how physical inactivity and sedentary behaviour have been rising over time, a phenomenon called the physical activity transition. It describes examples of how inactivity and sedentary behaviour harm the body and why they increase the risks of developing muscular skeletal conditions, type 2 diabetes, weight gain and obesity, cardiovascular disease and cancer. The chapter describes additional impacts of inactivity and sedentary behaviour in terms of mental health problems and wider economic costs. It explains how we measure physical inactivity, activity and sedentary behaviour, and presents recent epidemiological UK data. Data about travel are presented to illustrate the decline in activity in the UK. The UK Physical Activity Guidelines are explained along with guidelines for reducing sedentary behaviour. The chapter ends with a discussion on the benefits of increasing physical activity and reducing sedentary behaviour, for example how activity protects against type 2 diabetes, metabolic syndrome, cardiovascular disease, cancer and osteoporosis as well as promoting mental and emotional health.