  The Chern-Simons (CS) form evolved from an obstruction in mathematics into an
important object in theoretical physics. In fact, the presence of CS terms in
physics is more common than one may think: they seem to play an important role
in high Tc superconductivity and in recently discovered topological insulators.
In classical physics, the minimal coupling in electromagnetism and to the
action for a mechanical system in Hamiltonian form are examples of CS
functionals. CS forms are also the natural generalization of the minimal
coupling between the electromagnetic field and a point charge when the source
is not point-like but an extended fundamental object, a membrane. They are
found in relation with anomalies in quantum field theories, and as Lagrangians
for gauge fields, including gravity and supergravity. A cursory review of the
role of CS forms in gravitation theories is presented at an introductory level.
