The subject is reviewed with an emphasis on recent developments in model analyses. Application of computer technology has facilitated the study of simultaneous convection and diffusion within a complex geometry approximating the airways and air spaces of the lung. During an inspiration of fresh gas the diffusion front separating inspired from "alveolar" gas is stationed within a portion of the acinus where the magnitudes of gas transport by convection and diffusion are similar. This constitutes the peripheral boundary of the Fowler dead space. Asymmetry of intra-acinar branching results in an inequality of gas concentrations within alveolar gas, despite homogeneous volume expansion. This is due to an interaction between diffusion and convection at branch points subtending units of unequal lengths. During a respiratory cycle a "diffusive Pendelluft" between these units results in a "first in-first out" pattern and a sloping alveolar plateau on the expired tracing, despite synchronous and homogeneous volume changes. Experimental results suggest that incomplete alveolar gas mixing constitutes a measurable limitation to gas exchange in the normal lung.