The importance of coalescence for efficient application of extraction and reaction engineering involving liquid–liquid mixtures is reviewed, with an emphasis on the role of intensification techniques for improvement. Discussion of gravity settling is used as a starting point, which then extends into the role of solid surfaces, such as fibers, in promoting coalescence. The physical mechanisms which control interactions between drops that then lead to coalescence are then reviewed. These include collision frequency, interfacial drainage, the role of van der Waals forces, and dynamic changes in drop geometry. A brief introduction to population balance modeling for prediction of coalescence rates is presented. The application of electrostatics to intensification of coalescence is analyzed, with a short summary of the controlling relationships. Recent developments in the application of surfactants and electrolytes for the enhancement of coalescence are also reviewed, together with an overview of controlling equations. Other intensification techniques that are briefly reviewed include the application of ultrasonic fields, phase-inversion techniques, and the use of membranes.