Theoretical methods for computing intracell thermal-neutron densities, disadvantage factors and thermal utilizations are presented. The topics discussed are the computation of the neutron spectra in a lattice, anisotropic scattering by water, the cylindrical cell effect, correlation of spectral moments and average one-group cross sections, one-group transport-theory methods, scattering models of water, and comparison of various theories with experiment. The lattices investigated are water moderated with slightly enriched uranium metal and oxide fuel.