Publisher Summary This chapter describes the chromatographic band broadening technique. Mass transfer in gas chromatography takes place through diffusion across the gas stream, to the porous particles of support or adsorbent, then through diffusion in the stagnant gas which impregnates these particles and finally through diffusion in the liquid phase or adsorption-desorption on the gas-solid interface. As shown in the chapter, the contribution of the gas phase diffusion to the kinetics of mass transfer, and hence to the band broadening, is of major importance. Molecular diffusion plays a major role in both the radial and the axial direction of the column. In all these cases, the fundamental parameter that will control the kinetics is the diffusion coefficient. However, in elution chromatography, the sample is injected as a narrow plug into the mobile phase. Accordingly, there is a strong concentration gradient along the column axis, and following Fick's law of diffusion, a large flux of sample takes place in an opposite direction to the concentration gradient.