In chapter 5 we looked at TLM models of forced convection, being the mode by which heat is transferred between a fluid and a thermally conductive solid. This chapter looks at TLM models of forced convection from the point of view of the thermal processes within the fluid. There are many physical problems where a diffusion process is accompanied by a drift or overall movement of the diffusing species in one direction with respect to some fixed external reference. This chapter starts by outlining the basics of diffusion/ drift theory. We then describe some of the techniques that have been used in TLM implementations and conclude with a discussion on the stability and accuracy of diffusion/drift algorithms.