Just as doctors have led aspects of social change in the past, health professionals today must contribute to and lead actions on sustainability. Exponential growth and unsustainability can be observed in the global population, energy use, money supply and greenhouse gas emissions. As with all unsustainable systems, they will become sustainable, but the timing and manner are undecided and carry profound health threats. We are trapped using outmoded forms of thinking and by our cognitive dissonance as we consider these threats in the light of our own lifestyles. The aim should be a transition that will lessen inequalities, combat problems such as obesity, depression and addictive behaviours, and improve well-being. The challenge is similar to other major public health issues in that the problem needs to be identified, evidence gathered, theories developed, alliances built, policies formulated and actions taken. This paper outlines how this can be done but suggests that the response needed will be unprecedented, and calls for action on what is known and debate about what is uncertain.