Commitment contracts, whereby people deposit money that they receive back only if they succeed, have substantial conceptual appeal as a method of changing health behaviour. Scott Halpern, David Asch, and Kevin Volpp examine the evidence behind them and find many unanswered questions Much illness stems from poor health behaviours. But changing behaviours is difficult, particularly when immediate desires must be sacrificed to achieve future benefits,1 as when people try to quit smoking, eat less, or exercise more. To overcome these challenges, corporate ventures such as www.stickk.com and www.healthywage.com are banking, quite literally, on commitment contracts, offering the millions of people who struggle to lose weight or take their medicines more regularly the opportunity to deposit money that they will receive back only if they succeed. Grounded in behavioural economic theory,2 commitment contracts bring a risk of loss into the present, where the temptations also lie, and augment motivation to succeed. They potentially offer an efficient mechanism of behaviour change because people generally are more motivated to avoid losses than they are to achieve similarly sized gains.3