  We obtain the one-loop propagators in superstring theory for the general case
when the worldsheet fields satisfy non-trivial holonomy and/or boundary
conditions. Non-trivial holonomy arises in orbifold and orientifold backgrounds
whereas non-trivial boundary conditions arise in backgrounds containing
D-branes of different dimensionality or D-branes intersecting each other at an
angle. In our derivation, we use a generalized version of the method of images.
Dihedral groups play a crucial role in constructing the one-loop propagators.

High Energy Physics - Theory