  This is the fourth article in the collection of reviews "Exact results on N=2
supersymmetric gauge theories", ed. J.Teschner. It describes a very useful
mathematical representation of the results of the localisation computations of
instanton partition functions as integrals having a form familiar from the
study of matrix models. Techniques from the study of matrix models can be
employed to extract important information on the instanton partition functions
in various limits and special cases.
