Enhancement of mass transfer is one the most important factors in the intensification of liquid–liquid processes. The fundamentals of interphase mass transfer for single drops are reviewed, with summary of the important correlations developed to date. The cases of single oscillating drops and of droplets experiencing circulation are discussed with presentation of important correlations for mass-transfer coefficients. The discussion of single drops is extended to describe a quantitative approach to describing mass-transfer rates for liquid–liquid systems based on solution of the Navier–Stokes equations, continuity equations, and Fick’s law. The phenomena of time-dependent mass transfer, the role of interfacial instability, and Marangoni convection are described with presentation of the controlling equations. Comparisons between experimental mass-transfer data and predictions are shown. More complex cases involving swarming drops are considered, with a review of correlations for the calculation of mass-transfer rates in various continuous column contactors, including spray columns, pulsed packed columns, pulsed plate columns, and rotating disk columns.