According to the World Health Organisation (2018) mental health can be defined as "a state of wellbeing in which an individual realizes his or her own abilities, can cope with the normal stresses of life, can work productively and is able to make a contribution to his or her community". This definition acknowledges that mental health is more than the absence of mental disorders (Galderisi et al., 2015) and shows that sport can potentially create situations that challenge mental health where, for example, the athlete is unable to cope with the stresses of training and competition. As the quote from Kelly Holmes demonstrates athletes, like all members of society, can be affected by mental health difficulties. Mental health is perhaps best conceptualised as a continuum ranging from negative (ill health) to positive (good health/mental wealth; Kuettel & Larsen, 2019), where both clinically diagnosed mental health disorders and sub-clinical symptoms can have an impact on an individual's quality of life. Indeed, research has explored both clinical and sub-clinical symptoms in athletes and consequently throughout this chapter the term mental health difficulties is used to describe both clinically diagnosed mental health conditions and sub-clinical symptoms. The chapter explores the mental health of athletes by examining the prevalence of mental health difficulties in athletes, the mental health difficulties that can affect athletes, why athletes might be vulnerable to these, how they can be prevented or treated, and how positive mental health and wellbeing can be promoted in sporting environments. This will be further explored in Chapters 12-14 which investigate resilience, thriving, and athlete welfare which are all closely allied to mental health.