  We study topological boundary conditions in abelian Chern-Simons theory and
line operators confined to such boundaries. From a mathematical point of view,
their relationships are described by a certain 2-category associated to an even
integer-valued symmetric bilinear form (the matrix of Chern-Simons couplings).
We argue that boundary conditions correspond to Lagrangian subgroups in the
finite abelian group classifying bulk line operators (the discriminant group).
We describe properties of boundary line operators; in particular we compute the
boundary associator. We also study codimension one defects (surface operators)
in abelian Chern-Simons theories. As an application, we obtain a classification
of such theories up to isomorphism, in general agreement with the work of Belov
and Moore.
