Desorption is the removal of a volatile or semivolatile substance from a solid material. This chapter focuses on the desorption of organic contaminants from a solid or sludge. Thermal desorption is carrying out this process by the application of heat. Work carried out at the University of Utah by Lighty and co-workers was aimed at developing an understanding of the fundamental transport phenomena underlying the thermal desorption of organics from contaminated soils. Particle characterization reactors were used for the most fundamental studies, including exploration of intraparticle resistances in the soil. Another experimental device, a packed-bed system termed the bed characterization reactor, was constructed and operated to measure mass and heat transfer effects more accurately in real operating environments. Most studies on desorption of organics have been carried out on substrates from which the moisture had been removed prior to loading the organics onto the substrate.