In this chapter 'mental health problems' is used as an umbrella term, but 'mental illness' is sometimes used when discussing specific clinical diagnoses. Physical activity has been shown to have both preventative and therapeutic benefits for mental health. The mental health benefits of physical activity in the general population provide a rationale for therapeutic application in those with established mental health problems. Physical inactivity is one of the leading risk factors for global mortality with sedentary behaviour now recognised as a major public health issue. The benefits of being physically active are well documented and include the prevention and management of mental health conditions. The chapter introduces the scale of the challenge that mental health problems pose, globally and in the United Kingdom (UK), in terms of health, health care provision, and the economic burden. The multiple benefits of physical activity for mental, physical, and social health, and the absence of side-effects, make it an appealing adjunct or alternative to pharmacological treatments.