D-brane conformal field theory and bundles of conformal blocks

Conformal blocks form a system of vector bundles over the moduli space of complex curves with marked points. We discuss various aspects of these bundles. In particular, we present conjectures about the dimensions of sub-bundles. They imply a Verlinde formula for non-simply connected groups like PGL(n,C). We then explain how conformal blocks enter in the construction of conformal field theories on surfaces with boundaries. Such surfaces naturally appear in the conformal field theory description of string propagation in the background of a D-brane. In this context, the sub-bundle structure of the conformal blocks controls the structure of symmetry breaking boundary conditions.