  Ultrarelativistic particles called cosmic rays permeate the Milky Way,
propagating through the Galactic turbulent magnetic fields. The mechanisms
under which these particles increase their energy can be reasonably described
by current theories of acceleration and propagation of cosmic rays. There are,
however, still many open questions as to how to reach petaelectronvolt (PeV)
energies, the maximum energy believed to be attained in our Galaxy, and in
which astrophysical sources (dubbed {\it PeVatrons}) this ultra-high energy
acceleration happens. In this article, we describe the theoretical conditions
for plasma acceleration to these energies, and the Galactic sources in which
these conditions are possible. These theoretical predictions are then
confronted with the latest experimental results, summarising the
state-of-the-art of our current knowledge of PeVatrons. We finally describe the
prospects to keep advancing the understanding of these elusive objects, still
unidentified more than one hundred years after the discovery of cosmic rays.
