This chapter discusses liquid–liquid and gas–liquid systems together because of their similarity, and to avoid unnecessary repetition. The amount of the surfactant adsorbed per unit area of the liquid–liquid interface is usually obtained from surface or interfacial tension measurements using the Gibbs adsorption equation. The behavior of an emulsion also depends on the viscosity of liquid–liquid dispersion. Indirect methods used to measure drop size distribution in liquid–liquid dispersions are based on laser systems. Derksen and Van den Akker demonstrated the usefulness of direct numerical simulations of liquid–liquid dispersions in a three-dimensional periodic domain by means of the lattice Boltzmann method. Coalescence together with breakage controls drop and bubble size distributions in liquid–liquid and gas–liquid dispersions. The multifractal approach used for drop breakage is extended to the coalescence process. The chapter focuses on droplet coalescence in liquid–liquid dispersions; however, a similar approach can be applied to bubbles.