Obstructive sleep apnea (OSA) and type 2 diabetes are both closely related to obesity and their prevalence is increasing due to the rising average body weight in Western countries. The findings of epidemiological studies have implicated that OSA increases the risk for cardiovascular disease, and metabolic disturbances, such as insulin resistance, may link OSA to vascular morbidity. A number of observational clinical studies have evaluated the relationship between OSA and insulin resistance, suggesting an independent association. However, the confounding effect of obesity complicates the establishment of a causal relationship between OSA and insulin resistance. Potential mechanisms that may underpin this relationship were evaluated in animal and human experimental studies and include intermittent hypoxia, arousals from sleep with concomitant sympathetic activation and sleep fragmentation. Currently only three randomized controlled trials investigating the effects of OSA on insulin resistance have been published. In these trials OSA patients were randomly assigned to treatment with continuous positive airway pressure (CPAP) or subtherapeutic CPAP and treatment effects on various measures of insulin resistance were examined. In two of these trials there was no effect of CPAP on glucose metabolism and in one trial a small beneficial effect of CPAP was observed. Further carefully conducted clinical studies and randomized controlled interventional CPAP trials are needed to determine the extent to which OSA is a risk factor for diabetes and its effect on glucose metabolism.