Tracheal intubation for airway control, once done only by anesthesiologists during surgical procedures, is now being done by physicians in other specialties and by nurses, technicians, and paramedics in areas other than the operating room. Intubation, however, does not always assure a patent airway. Unrecognized esophageal placement of endotracheal tubes is the major cause of cardiac arrest and brain damage associated with intubation. Though auscultation for breath sounds is the universally accepted method of verifying proper tube placement, recent studies indicate that it is reliable only approximately two thirds of the time in situations in which verification of proper placement is needed most and is least obvious. The usefulness of this technique merits reassessment. Identification of carbon dioxide in end-expired air is the most reliable method for verification, but instruments to detect carbon dioxide are usually immediately available only in special care and surgical suites. Mouth-to-tube insufflation with a two-way disposable microbial filter differentiates immediately between esophageal and tracheal placement and can be used in any area. Malpositioned and malfunctioning tubes cause partial or complete obstruction accompanied by varying degrees of hypoxemia and hypercapnia. Respiratory and circulatory derangements and brain damage ensue if the problem is not promptly recognized and corrected. We discuss the most common causes of tube malfunction.