  This is the 8th article in the collection of reviews "Exact results in N=2
supersymmetric gauge theories", ed. J. Teschner. The article reviews the
superconformal index. It is often simpler to calculate than instanton partition
functions, but nevertheless allows one to perform many nontrivial checks of
conjectured dualities. It turns out to admit a representation in terms of a new
type of topological field theory associated to the Riemann surfaces $C$
parameterising the class $\cal S$ theories.
