The quote by Paul Jarvis, which he offered in a synopsis paper concerning leaf diffusive resistances, contains implicit reference to the fact that controls over plant-atmosphere fluxes reflect not only the processes that drive the exchanges of H2O and CO2, but also past evolutionary modification of the leaf form and function. Thus, an understanding of fluxes at the plant and leaf scales requires perspectives on adaptation, in addition to biophysical processes. In fact, recognition that leaf and plant function can be best explained when both of these principles are integrated into a common framework has served as the intellectual cornerstone for the discipline of plant physiological ecology for over four decades. In this chapter we develop this integration with regard to the specific case of leaf processes and their underlying diffusive fluxes. In the next chapter, we will consider explicitly the process of adaptation with regard to leaf function, and the concept of adaptation as an organizing principle from which we can predict patterns of covariance between environmental change and traits that control leaf-atmosphere gas exchanges. Although we will focus on the leaf scale in both chapters, we will also begin to introduce concepts associated with atmospheric pressure gradients, turbulent transport, and eddy diffusivity, all of which will be valuable as we move into future chapters.