  The observation of long-range collective correlations for particles emitted
in high-multiplicity pp and pPb collisions has opened up new opportunities of
investigating novel high-density QCD phenomena in small colliding systems. We
review experimental results related to the studies of collective phenomena in
small systems from RHIC and the LHC over the past several years. Latest
development in theoretical interpretations motivated by different frameworks
are also reviewed, and confronted with the experimental data. Perspectives on
possible future directions are discussed, with the aim of further exploring the
rich emergent QCD phenomena.
