Fire suppression with water mist is widely considered to be an alternative to gaseous fire suppression agents. Much commercial activity has occurred in the last 5 years to develop technology for fire suppression systems based on water mist. Although researchers in the 1950's recognized the dominant mechanisms of extinguishment, recent experimental work has improved the understanding of the extinguishing properties of water mist. This paper proposes a classification terminology for water mists to facilitate discussion of water mist systems. It describes the primary mechanisms of extinguishment, namely heat extraction, oxygen displacement, and radiation attenuation, and invokes theoretical considerations of vapor/air mixture dilution and kinetic effects at the molecular level. Evidence of an effect of invigoration of combustion caused by the introduction of water mist is presented. Not all fires are extinguished rapidly by water mist: the time to extinguish a fire is a function of fuel type, geometry and mist characteristics. An understanding of the suppression properties of water mist allows a designer to make reasonable choices in designing a suppression system for a particular fire scenario, and is essential for the development of algorithms for computer models of water mist suppression systems.