‘Humidification, Nebulization, and Gas Filtering’ describes the therapies, which, while not essential to ventilation, are frequently applied together with it. This chapter provides a comprehensive discussion of these devices, including the clinical needs for them and their operating principles, options, and common complications. It describes the composition and operating principle of active heated humidifiers. Artificial humidification brings with it the additional issue of circuit rainout, which may require that heated wires be added to the breathing circuit. The chapter includes a table for troubleshooting issues related to active humidification. Heat and moisture exchangers or HMEs, an alternative to humidifiers, are also discussed. Finally, the chapter turns to the use of in-line nebulizers or aerosol therapy. The chapter ends with a discussion of respiratory gas filtering, which, for all the benefit a bacteria filter brings, also introduce the risk of occlusion.