A fog machine or smoke machine is a device which emits a dense vapour that appears similar to fog or smoke. This artificial fog is most commonly used in professional entertainment applications, but smaller, more affordable fog machines are becoming common for personal use. Fog machines can also be found in use in a variety of industrial, training, and some military applications. Typically, fog is created by vapourizing proprietary water and glycol-based or glycerine-based fluids or through the atomization of mineral oil. This fluid (often referred to colloquially as fog juice) vapourizes or atomizes inside the fog machine. Upon exiting the fog machine and coming into contact with the moisture in the outside air the vapour condenses, resulting in a thick visible fog.

It is recommended using the manufacturer's fog fluid. Fog machines are calibrated for temperatures of the fluid formulated for that particular model. Using incorrect fluid for the fog machine could result in a damaged machine due to overheating during the flashing process.