of an eigenvalue nature. There long has been a curious "folk history" effect in this area~ Wigner and Weinberg could de­ scribe it as "what was generally known was generally untrue". This account of the Boltzmann equation surely will show that a rigorous basis for our expectations of certain solutions can be well-founded on analysis. Ely Gelbard's review of the methods of determining diffusion-type parameters in complex geometries where simple diffusion theory would be welcome has required just as much rigor to determine how such modeling can be made accurate, although to a more immediate and practical purpose. The two articles can be seen as interesting contrasts, facets of the same underlying problem showing apparently different aspects of the same central core.