The principles of mass transfer are founded on the three themes common to most reactor problems: (1) a materials balance expression for the reactor, (2) reaction equilibrium, and (3) transport kinetics. For gas transfer, these themes are delineated in terms of materials balance (Section 4.3), Henry’s law (Appendix H), turbulent mixing (Section 184.108.40.206 and Chapter 10) and Fick’s first law (Section 220.127.116.11), respectively. The approach of this chapter is to apply such fundamentals as the foundation for operational equations. Operational equations differ from one another by the characteristics of the particular application, whether surface aerators for activated sludge, diffused aeration for activated sludge, packed towers for air stripping, or stream reaeration. The differences are due essentially to the means in which interfacial surface area is created, its rate of creation, and the turbulence scale and intensity.