  In the late 1980s Witten used the Chern-Simons form of a connection to
construct new invariants of 3-manifolds and knots, recovering in particular the
Jones invariants. Since then the associated topological quantum field theory
(TQFT) has served as a key example in understanding the structure of TQFTs in
general. We survey some of that structure with a particular focus on the
"multi-tier" aspects. We discuss general axioms, generators-and-relations
theorems, a priori constructions, dimensional reduction and K-theory, and
Chern-Simons as a 0-1-2-3 theory. An appendix gives a lightening treatment of
the Chern-Simons-Weil theory of connections. The paper concludes with general
remarks about the Geometry-QFT-Strings interaction.
