  Correlations between particles separated by several units of pseudorapidity
were discovered in high-multiplicity pp and p-Pb collisions at the LHC. These
long-range structures observed in two-particle correlation functions are
reminiscent of features seen in Pb-Pb collisions, where they are often viewed
as a signature of collective behavior and the formation of a quark-gluon plasma
(QGP). Therefore, the discovery of these `ridges' in small systems has
implications for the study of collectivity in small systems as well as in
heavy-ion collisions. The ridges in pp and p-Pb collisions have been studied in
the ALICE, ATLAS, CMS, and LHCb experiments to characterize the $p_{\rm{T}}$-,
$\eta$-, and multiplicity-dependences of the ridge yield, as well as its
particle composition.
