This paper is an elementary introduction to particle diffusion in a medium where the coefficient of diffusion varies with position. The introduction is aimed at third-year university courses. We start from a simple model of particles hopping on a discrete lattice, in one or more dimensions, and then take the continuous-space limit so as to obtain the differential equation of diffusion. The discrete-step model is next adapted to investigate diffusion by continuous-length steps; the coefficient of diffusion is related to the mean free path. In both the discrete- and continuous-step models, the diffusion law has the Fokker-Planck (instead of Fick) pattern if the jumps are not biased and occur equally likely in all directions. Lastly, three experimental examples of diffusion in inhomogeneous media are examined; the observed law can be of a Fokker-Planck, Fick or hybrid type. (Contains 5 footnotes and 3 figures.)