Processes involving the growth, motion and coalescence of fluid droplets have been studied for many years because of their scientific interest and practical importance. Interest in these processes has also been stimulated because they provide simple and often quite beautiful examples of pattern formation phenomena. In recent years it has been shown that a quite wide variety of processes in which droplet coalescence plays a major role exhibit simple asymptotic scaling behavior. These observations have stimulated renewed interest in experimental investigations, computer simulations and theoretical work. In many cases droplet growth and coalescence processes can be described in terms of quite simple models and this makes theoretical work and computer simulations quite attractive. One of the main examples is the condensation of water vapour on a cold surface (breath figures or dropwise condensation). All four stages in the breath figure process (1, droplet growth without coalescence, 2, droplet growth and coalescence without renucleation, 3, growth and coalescence with renucleation and 4, growth, coalescence and renucleation with large droplet removal) can be described in terms of simple computer models and scaling ideas. Recent advances in the study of breath figures and related phenomena is the main focus of this review. Other processes are described in less detail to illustrate the breadth of application of these approaches to phenomena involving droplet coalescence.
