Book excerpt: A major new textbook with strong international appeal and the first to bring together research in this fascinating and important area, it has the potential to become the standard text for a very large market One in four adults experiences mental health problems. The health cost of this is massive and there’s now significant interest in activity and exercise therapy from government down. Follows on from Routledge's successful publications Psychology of Physical Activity (Biddle & Mutrie) and Physical Activity and Psychological Wellbeing (Biddle, Fox et al)