In this paper I first briefly look at diffusion in several controlled
release devices. I have found that these exploit complicated diffusion
al mechanisms. These cannot be understood using the conventional descr
iption of diffusion. So, a general theory of multicomponent diffusion
which can describe them is considered. An overview follows of the pres
ent knowledge on diffusivities of small and large permeants in dense p
olymers, swollen polymers, charged polymers, gels and porous media. Th
e paper ends with a brief discussion of the limitations of diffusion t
heory in swelling polymers.