  This is the seventh article in the collection of reviews "Exact results on
N=2 supersymmetric gauge theories", ed. J.Teschner. It discusses an interesting
class of observables localised on surfaces that attracts steadily growing
attention. In the correspondence to conformal field theory some of these
observables get related to a class of fields in two dimensions called
degenerate fields. These fields satisfy differential equations that can be used
to extract a lot of information on the correlation functions. Understanding the
origin of these differential equations within gauge theory may help explaining
the AGT-correspondence itself.

High Energy Physics - Theory