Drilling fluids are generally emulsions, either oil-in-water (O/W) or water-in-oil (W/O), with an aqueous phase containing various electrolytes and an oil phase ranging from hydrocarbons to polar oils. Such multiphasic systems are stabilized by surfactants and co-surfactants, whose nature is related to the emulsion morphology. Stable emulsions should have an unbalanced generalized formulation, which is also related to the phase behavior at equilibrium. If the drilling fluid tends to form a viscous multiphase in the casing void or enters the porous medium and plugs it by capillarity, it has to be removed as a single-phase system of the mesophase type. This usually requires the formation of a bicontinuous microemulsion with a surfactant system exhibiting the highest solubilization of both oil and water phases, a situation that takes place when the formulation is exactly balanced. Moreover, this should also occur with an amount of surfactant as small as possible, and this is yet an unsolved problem. Recent developments indicate that there are several trends toward performance improvement that can be used in parallel or together, and this is what has been used in practical cases.