  This contribution identifies and characterizes a previously unrecognized
boundary--layer structure that occurs in the context of linear transport
theory, with an impact on the fields of Radiative Transfer and Neutron
Transport. The existence of this boundary layer structure, which governs the
interactions between different materials, or between a material and vacuum,
plays a critical role in a correct description of transport phenomena.
Additionally, the boundary--layer phenomenology explains and helps bypass
computational difficulties reported in the literature over the last several
decades.
