Obesity is a heterogeneous disease that can develop via slow and steady weight gain over an extended period, or from rapid bursts of weight gain. Regular assessments of body weight are needed to catch early weight gain. Use the Edmonton Obesity Staging System to evaluate if the patient has obesity. Clinicians should initiate discussion around weight gain early and contemplate interventions that consider its com­plex causes, providing guidance beyond “eat less and move more.” Many medications are associated with weight gain side effects that can contribute to long-term weight gain. Excess pregnancy weight gain and post pregnancy weight retention are significantly reduced with behavioural interventions. Clinicians should counsel women attending prenatal care not to exceed pregnancy weight gain guide­lines, and also give pregnant women the necessary coun­selling, as well as dietary, physical activity and psychological interventions within prenatal visits. Health benefits of smoking cessation outweigh the cardio­vascular consequences associated with smoking cessation related weight gain. Short-term behavioural interventions (generally six months or less) aimed at preventing weight gain in young adult­hood, menopause, smoking cessation and breast cancer treatment have not yet been shown to be effective. Longer interventions will likely be needed to properly examine strategies for preventing weight gain for many of these high-risk groups and in the general population.