A fire and explosion suppression system comprises a source (5) of high pressure water which is fed to a misting nozzle (13) at one input of a mixing unit (6), and a source (14) of high pressure inert gas, such as nitrogen, which is fed along a pipe (20) to another input of the mixing unit (6). Inside the mixing unit (6), water mist, in the form of an atomized mist of very small droplet size is mixed with the pressurized gas and exits the mixing unit (6) at high pressure and high velocity along a pipe (22) and is thence discharged through spreaders (26, 28). The source (5) of the water is pressurized by a feed (30) from the source of pressurized inert gas. The mass flow rate of the water will therefore reduce as the pressure of the gas decays. This tends to maintain the ratio of the mass flow rate of the water to the mass flow rate of the gas constant. This is found to produce and maintain an advantageous distribution of droplet size in the discharged unit. A control unit (10) adjusts a metering valve (7) in dependence on the mass flow rate or the pressure of the gas in order to adjust the ratio as necessary to maintain its value constant.