  The recent observation of an energy structure in the reactor antineutrino
spectrum is reviewed. The reactor experiments Daya Bay, Double Chooz and RENO
have reported a consistent excess of antineutrinos deviating from the flux
predictions, with a local significance of about 4$\sigma$ between 4 and 6 MeV
of the positron energy spectrum. The possible causes of the structure are
analyzed in this work, along with the different experimental approaches
developed to identify its origin. Considering the available data and results
from the three experiments, the most likely explanation concerns the reactor
flux predictions and the associated uncertainties. Therefore, the different
current models are described and compared. The possible sources of
incompleteness or inaccuracy of such models are discussed, as well as the
experimental data required to improve their precision.
