Inhaled therapy is commonly employed in mechanically ventilated patients with chronic obstructive pulmonary disease or asthma. The efficacy of inhaled drugs is comparable to that achieved with systemic routes of administration, but the dose of drug required to achieve a therapeutic effect is generally much smaller. Moreover, limited systemic absorption of inhaled drugs minimises systemic side effects. Aerosol administration to ventilated patients differs from that in ambulatory patients in several respects. Optimal techniques for using pressurised metered-dose inhalers and nebulisers in ventilator circuits have been developed. With these techniques, the efficiency of inhaled drug delivery in mechanically ventilated patients is now comparable to that in ambulatory patients. Pressurised metered-dose inhalers are chiefly used to deliver bronchodilator and corticosteroid aerosols, and are more efficient and convenient to use than nebulisers for routine therapy in ventilated patients. However, nebulisers are more versatile and are employed to generate aerosols of bronchodilators, corticosteroids, antibiotics, prostaglandins, surfactant and mucolytic agents. Improvements in drug formulations and the design and efficiency of aerosol generating devices have led to increasing application of inhaled therapies in mechanically ventilated patients.