  We review work on `decomposition,' a property of two-dimensional theories
with 1-form symmetries and, more generally, d-dimensional theories with
(d-1)-form symmetries. Decomposition is the observation that such quantum field
theories are equivalent to (`decompose into') disjoint unions of other QFTs,
known in this context as "universes." Examples include two-dimensional gauge
theories and orbifolds with matter invariant under a subgroup of the gauge
group. Decomposition explains and relates several physical properties of these
theories -- for example, restrictions on allowed instantons arise as a
"multiverse interference effect" between contributions from constituent
universes. First worked out in 2006 as part of efforts to resolve technical
questions in string propagation on stacks, decomposition has been the driver of
a number of developments since. We give a general overview of decomposition,
describe features of decomposition arising in gauge theories, then dive into
specifics for orbifolds. We conclude with a discussion of the recent
application to anomaly resolution of Wang-Wen-Witten in two-dimensional
orbifolds. This is a contribution to the proceedings of the conference
Two-dimensional supersymmetric theories and related topics (Matrix Institute,
Australia, January 2022), giving an overview of a talk given there and
elsewhere.
