This review presents the current status on the theory and mechanism of bubble coalescence and experimental data on coalescence time and bubble size, wherein coalescence phenomena are discussed. The various physical processes involved in bubble coalescence are identified and their contributions in the case of pure liquids, electrolytes and surfactants solutions discussed. The theoretical models proposed on the basis of different mechanisms have been reviewed and their utility in application to practical systems critically discussed. In the light of this theory, the available experimental data on coalescence time and bubble size and the role of electrolytes and surfactants are discussed. The future scope of work and possible approaches for correlation of the data are presented.