Continuous positive airway pressure (CPAP) is a method of respiratory ventilation used primarily in the treatment of sleep apnea and various lung diseases. CPAP ventilation can also be utilised during the "weaning off" off period of a ventilated intensive care patient, but this will not be covered in this article.

A device called a CPAP machine, which is basically a pump, is used to pass a controlled stream of air into the airway of the sleeping patient, splinting the airway (keeping it open under air pressure) so that unobstructed breathing becomes possible, reducing and/or preventing apneas and hypopneas.

CPAP treatment is highly effective in treating obstructive sleep apnea, although sometimes only partial improvement results. The other recognised type of sleep apnea (caused by central apnea) cannot be treated using CPAP.

Prospective CPAP candidates are often reluctant to use this therapy, since the face mask and hose to the machine looks uncomfortable and clumsy, and the airflow required for some patients can be vigorous. In spite of this, a few minutes of use is often all that is needed to become acclimated to the device. The improvement in the quality of sleep due to CPAP treatment will often be noticed after a single nights' use.

Recent CPAP machines are smaller, lighter, and much less noisy than older versions. Few airport inspectors have not seen the portable machines, so travel with them is not a problem. Air humidifiers and supplementary oxygen can be easily added to the input port of most CPAP machines, and some versions can also adapt to patient needs by monitoring patient breathing patterns.