To describe the behavior of an open system, we will consider an approach in which the spatial domain is considered to be finite, corresponding to the extent of the system, and boundary conditions are applied which permit particles to pass into and out of the system. The first model we will consider employs time-reversible boundary conditions which are plausible, but which we will ultimately see to be unphysical (Frensley, 1985). This model helps to define the conditions that a physically reasonable open-system model must display.