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.
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