Stochastic difference equations and a stochastic partial differential
equation (SPDE) are simultaneously derived for the time-dependent neutron
angular density in a general three-dimensional medium where the neutron angular
density is a function of position, direction, energy, and time. Special cases
of the equations are given such as transport in one-dimensional plane geometry
with isotropic scattering and transport in a homogeneous medium. The stochastic
equations are derived from basic principles, i.e., from the changes that occur
in a small time interval.