In synthesis, processing and applications of aerogels transport of liquids and gases in and through a wet gel or aerogel quite often determine the time scale and the properties. The transport of molecules is a diffusive process, meaning that the molecules move randomly in the solvent biased by concentration or chemical potential gradients. They typically diffuse from points of higher potential (mostly also higher concentration) to regions of lower potential or concentration. Similarly, a diffusion process occurs when a wet gel is washed, for instance, in an ethanol bath to exchange the pore fluid after gelling and ageing to one which is miscible with, for instance, carbon dioxide. In such a situation, the wet gel is overlaid by ethanol, which then diffuses into the pore space. Inasmuch as it diffuses inwards, the gel fluid moves outwards into the ethanol layer. During adsorption and desorption studies on aerogels, nitrogen diffuses into the pore system and will then be adsorbed or desorbed there. This transport takes time, and wediscuss characteristic times for such a process. This chapter discusses concepts of diffusion of species in general and in aerogels especially.