Gas–liquid multiphase flows play an essential role in the workings of Nature and the enterprises of mankind. Our everyday encounter with liquids is nearly always at a free surface, such as when drinking, washing, rinsing, and cooking. Similarly, such flows are in abundance in industrial applications: heat transfer by boiling is the preferred mode in both conventional and nuclear power plants, and bubble driven circulation systems are used in metal processing operations such as steel making, ladle metallurgy, and the secondary refining of aluminum and copper. A significant fraction of the energy needs of mankind is met by burning liquid fuel, and a liquid must evaporate before it burns. In almost all cases the liquid is therefore atomized to generate a large number of small droplets and, hence, a large surface area. Indeed, except for drag (including pressure drops in pipes) and mixing of gaseous fuels, we would not be far off to assert that nearly all industrial applications of fluids involve a multiphase flow of one sort or another. Sometimes, one of the phases is a solid, such as in slurries and fluidized beds, but in a large number of applications one phase is a liquid and the other is a gas. Of natural gas–liquid multiphase flows, rain is perhaps the experience that first comes to mind, but bubbles and droplets play a major role in the exchange of heat and mass between the oceans and the atmosphere and in volcanic explosions.