Pressurized metered-dose inhalers (pMDIs) and nebulizers are routinely employed for aerosol delivery in mechanically ventilated patients. A significant proportion of the aerosol deposits in the ventilator circuit and artificial airway, thereby reducing the inhaled drug mass. Factors influencing aerosol delivery during mechanical ventilation differ from those in spontaneously breathing patients. The English language literature on aerosol delivery during mechanical ventilation was reviewed. Marked variations in the efficiency of drug delivery with pMDIs and nebulizers occur due to differences in the technique of administration. Careful attention to five factors, viz., the aerosol generator, aerosol particle size, conditions in the ventilator circuit, artificial airway, and ventilator parameters, is necessary to optimize aerosol delivery during mechanical ventilation. Factors influencing drug delivery during NPPV are not well understood, and the efficiency of aerosol delivery in this setting is lower than that during invasive mechanical ventilaiton. With an optimal technique of administration the efficiency of aerosol delivery during mechanical ventilation is similar to that achieved during spontaneous breathing. Further research is needed to optimize aerosol delivery during NPPV.