Inhalation is a potentially viable route of insulin administration. This treatment is being tested but has not been approved by the United States Food and Drug Administration (FDA). The lung airways contain bronchial tubes, which are impermeable to insulin and alveoli, from which insulin can be absorbed into the circulation. Inhaled asthma medications deposit before reaching the alveoli. Novel devices can deliver insulin particles via slow even breaths into the alveoli. An aerosol of either powdered or solubilized insulin can be released mechanically or electronically. Pulmonary toxicity due to inhaled insulin has not been reported, but no chronic use studies have been conducted. The efficiency of insulin delivery by inhalation may be compromised by: (1) losses within the device and the environment; (2) deposition onto the throat and bronchial tubes; and (3) incomplete absorption from the alveoli. With current delivery technology, the maximum possible delivery efficiency is approximately 30%. The peak activity of inhaled regular insulin occurs at 60 minutes, which is similar to that of subcutaneous lispro insulin. Inhaled long-acting insulin is in early development. Studies comparing sequentially administered subcutaneous and inhaled regular insulins have demonstrated significantly reproducible glucose-lowering effects of inhaled insulin. In type 1 and insulin-treated type 2 patients, substitution of premeal inhaled insulin for subcutaneous insulin has resulted in no significant change in control. In a series of patients with type 2 diabetes failing oral agents, addition of premeal inhaled insulin resulted in significantly improved control. Inhaled insulin will become established if ongoing studies continue to demonstrate this technology to be safe and effective.