The theory of space-dependent stochastic fluctuations is developed in sufficient generality that any specialization can be made to a particular reactor model by finding the appropriate Green's function for the mean-neutron-density equation of the system in question. The approach used is the Langevin technique which, as developed here, yields the cross-correlation function as a double convolution over two Green's functions and the correlation function of equivalent “noise sources” present within the system. The character of these noise sources is examined in considerable detail to gain the basic physical understanding necessary to arrive at a calculational procedure and specific formulae. It is shown that when delayed-neutron effects are included, the input noise sources are not white. That is, their spectral-density functions are not constant. A clear distinction is made between fluctuations in the neutron density and the fluctuations observed with a detector. The density fluctuations include contributions from a neutron correlated with itself and direct progeny, whereas the mechanism of detection (invariably removing a neutron) eliminates this correlation.