The propagation of neutron waves through homogeneous nuclear systems, multiplying or non-multiplying, is studied with the aid of the general linear model. This model is characterized by a relationship between the complex wave length and frequency, a dispersion law. It is shown that, independent of the geometry of the system, the nature of the propagation and hence the neutron wave optics of the medium, is governed by this dispersion law. It is also shown how this dispersion law can be measured in the general situation, using spectral analysis and modal decontamination techniques. When specialized to particular geometries, but not to particular systems, the possibility of stop-and-pass frequencies emerges. When specialized still further to a multiplying system governed by age-diffusion theory, a new criterion for criticality is found. This latter should be of interest in monitoring the approach to critical condition in a large reactor whose kinetics are spatially dependent.