The high attrition rates in obesity interventions are associated with poorer weight loss and maintenance for the individual and poorer overall treatment effectiveness and cost-effectiveness for the treatment provider. Increased knowledge about factors associated with attrition can facilitate the identification of individuals at risk of drop-out and inform treatment program improvements with the aim of maximising treatment retention. To date, a relatively small body of literature has explored attrition from weight-loss interventions using two methods of attrition assessment: identification of pre-treatment predictors of attrition and eliciting post-treatment reasons for attrition. A range of attrition rates have been reported and no reliable or consistent predictors of attrition have been found. It is unknown whether the lack of consistent findings reflects population or treatment differences, or if the discrepant findings simply reflect differences in definition and measurement of attrition. Further research is required to address these limitations. There is a need for a recognised definition of obesity treatment attrition, the consideration of predictors that are theoretically and empirically associated with attrition, the development of a well-validated and standardised measure of barriers to attendance, and assessment of both treatment completers and drop-outs. Understanding the factors that influence attrition can be used to inform the modification of treatment programs and to target those most at risk of drop-out so as to maximise the success of obesity interventions.