  We develop the general form of the variational multiscale method in a
discontinuous Galerkin framework. Our method is based on the decomposition of
the true solution into discontinuous coarse-scale and discontinuous fine-scale
parts. The obtained coarse-scale weak formulation includes two types of
fine-scale contributions. The first type corresponds to a fine-scale volumetric
term, which we formulate in terms of a residual-based model that also takes
into account fine-scale effects at element interfaces. The second type consists
of independent fine-scale terms at element interfaces, which we formulate in
terms of a new fine-scale "interface model". We demonstrate for the
one-dimensional Poisson problem that existing discontinuous Galerkin
formulations, such as the interior penalty method, can be rederived by choosing
particular fine-scale interface models. The multiscale formulation thus opens
the door for a new perspective on discontinuous Galerkin methods and their
numerical properties. This is demonstrated for the one-dimensional
advection-diffusion problem, where we show that upwind numerical fluxes can be
interpreted as an ad hoc remedy for missing volumetric fine-scale terms.
