The influence of heterogeneity on neutron spectra in nuclear reactors and assemblies was investigated. The heterogeneous reactor model, a principal classification of neutron spectra, slowing down spectra in a plate-type reactor, equilibrium spectra in a plate-type reactor, reactor spectra in the thermal range, the represertation of reactor spectra by simple sets of functions, and the iterated multigroup method are discussed. Graphs are included. (M.C.G.)