||Obstructive sleep apnea (OSA) is a common disorder characterized by recurrent episodes of partial (hypopnea) or complete (apnea) upper airway obstruction during sleep despite ongoing respiratory efforts, resulting in disruption of sleep (arousal). OSA affects 9% of middle-aged men and 3% of women in North America. If left untreated, OSA can lead to fatigue, somnolence, headaches, cardiovascular disease, decreased quality of life, and increased risk of motor vehicle accidents. The gold standard assessment for OSA is polysomnography, a test that measures neurologic and cardio-respiratory parameters during sleep. The frequency of obstructive events measured during polysomnography is reported as the apnea-hypopnea index (AHI). According to the American Academy of Sleep Medicine, the severity of OSA is defined by the following AHI cut-offs: mild, e 5 and <15 events/hour; moderate, e 15 and <30 events/hour; severe, e 30 events/hour. OSA is often accompanied by clinical symptoms such as excessive daytime sleepiness, which is most frequently assessed using the Epworth Sleepiness Scale (ESS). The ESS is a questionnaire that has participants rate his or her likelihood of falling asleep in eight different daily situations on a scale of 0 to 24, with higher scores indicating greater sleepiness (ESS d 7, normal sleepiness). The most commonly used objective measures of daytime sleepiness are the Maintenance of Wakefulness Test (MWT), which measures the capacity to stay awake in conditions ideal for falling asleep, and the Multiple Sleep Latency Test (MSLT), which measures the tendency to fall asleep in favourable conditions. Treatment options for OSA include weight loss, dental devices or oral appliance therapy, surgical procedures, and continuous positive airway pressure (CPAP). CPAP is the mainstay of medical treatment for OSA and involves the use of a pump to deliver air into the nose or mouth via a mask during sleep. Positive pressure is generated by the airflow, which opens up the airway and prevents the soft tissue from collapsing. The effectiveness of CPAP is often limited by poor adherence rates, underscoring the importance of identifying barriers to adherence and developing tailored interventions to improve adherence. OSA has a substantial economic impact due to an increased risk of cardiovascular disease, decreased quality of life, increased risk of motor vehicle accidents, and loss in occupational productivity. Studies evaluating the effect of CPAP on medical costs are limited and findings are not consistent. The purpose of this review is to examine the clinical evidence, cost effectiveness and guidelines regarding the use of CPAP treatment for adults with moderate to severe OSA.