Physicians are familiar with conventional nebulizers, which deliver aerosols in a relatively uncontrolled manner. As aerosol medications evolve beyond bronchodilators, the need for control of dose variability, the possibility of overdose, and the need for efficient delivery have provoked the industry to redesign aerosol delivery systems. The need to target aerosol delivery to specific lung regions has focused efforts to coordinate aerosol delivery with defined breathing maneuvers. This review summarizes the major factors affecting aerosol deposition, discusses how those factors are guiding new designs for aerosol delivery systems, and describes some examples of the improved precision and efficiency of those systems.