The interaction between obesity and obstructive sleep apnea (OSA) is complex. Whilst it is often assumed that obesity is the major cause of OSA, and that treatment of the OSA might mitigate further weight gain, new evidence is emerging that suggest these statements may not be the case. Obesity explains about 60% of the variance of the apnea hypopnea index definition of OSA, mainly in those < 50 years and less so in the elderly. Moreover, long term treatment of OSA with continuous positive airway pressure is associated with small but significant weight gain. This weight gain effect may result from abolition of the increased work of breathing associated with OSA. Weight loss, by either medical or surgical techniques, which often cures type 2 diabetes, has a beneficial effect upon sleep apnoea unfortunately in a minority of patients. A short jaw length may be predictive of a better outcome. The slight fall in the overall apnea hypopnea index with weight loss, however may be associated with a larger drop in the non-supine apnea hypopnea index, thus converting some patients from non-positional to positional (ie supine only) OSA. Importantly, patients undergoing surgical weight loss need close monitoring to prevent complications. Finally, in patients with moderate to severe obesity related OSA, the combination of weight loss with CPAP appears more beneficial than either treatment in isolation.