  We review the properties of orbifold operations on two-dimensional quantum
field theories, either bosonic or fermionic, and describe the "Orbifold
groupoids" which control the composition of orbifold operations.
Three-dimensional TQFT's of Dijkgraaf-Witten type will play an important role
in the analysis. We briefly discuss the extension to generalized symmetries and
applications to constrain RG flows.
