  The study of angular correlations of heavy-flavor particles in hadronic
collisions can provide crucial insight into the heavy quark production,
showering, and hadronization processes. The comparison with model predictions
allows us to discriminate among different approaches for heavy quark production
and hadronization, as well as different treatments of the underlying event
employed by the models to reproduce correlation observables. In
ultra-relativistic heavy-ion collisions, where a deconfined state of matter,
the quark--gluon plasma (QGP), is created, heavy-flavor correlations can shed
light on the modification of the heavy quark fragmentation due to the
interaction between charm and beauty quarks with the QGP constituents, as well
as characterize their energy loss processes while traversing the medium.
Insight into the possible emergence of collective-like mechanisms in smaller
systems, resembling those observed in heavy-ion collisions, can also be
obtained by performing correlation studies in high-multiplicity proton--proton
and proton--nucleus collisions. In this review, the most recent and relevant
measurements of heavy-flavor correlations performed in all collision systems at
the LHC and RHIC will be presented, and the new understandings that they
provide will be discussed.
