This chapter contains section titled: Introduction Factors Influencing the Safety and Efficacy of Drugs Delivered from Pressurized Metered‐Dose Inhalers Hydrofluoroalkane and Chlorofluorocarbon Pressurized Metered‐Dose Inhalers Active Drug Propellants Co‐Solvents Surfactants Other Excipients Mechanical Factors Influencing Drug Delivery Spacers Patient Factors Dry Powder Inhalers Nebulizers Conclusions References Introduction Factors Influencing the Safety and Efficacy of Drugs Delivered from Pressurized Metered‐Dose Inhalers Hydrofluoroalkane and Chlorofluorocarbon Pressurized Metered‐Dose Inhalers Active Drug Propellants Co‐Solvents Surfactants Other Excipients Mechanical Factors Influencing Drug Delivery Spacers Patient Factors Dry Powder Inhalers Nebulizers Conclusions References