Abstract Emulsion droplet size plays a key role in many emulsion properties such as stability, color, appearance, texture, and rheology. Accordingly, different emulsions have been classified based on emulsion size including microemulsions and submicron (nano) emulsions. The aim of emulsification is usually to produce emulsion droplets as small as possible, and various emulsification techniques can be used in this regard. One of the main problems of producing very fine emulsions with high-energy emulsification techniques such as microfluidization is the occurrence of “over-processing”, which is increase in emulsion size by supplying more energy due to a high rate of re-coalescence of new droplets. Different reasons have been mentioned for “over-processing”, including low adsorption rate of the surface-active agent, low residence time of the emulsion in the emulsification zone, high rate of coalescence frequency, and extreme amount of energy density. This review highlights re-coalescence of new droplets during high-energy emulsification along with some common and important emulsification techniques and different factors affecting emulsion droplet size, and thereby re-coalescence.