  Among the emission-line stars, the classical Be stars known for their extreme
properties are remarkable. The Be stars are B-type main sequence stars that
have displayed at least once in their life emission lines in their spectrum.
Beyond this phenomenological approach some progresses were made on the
understanding of this class of stars. With high-technology techniques
(interferometry, adaptive optics, multi-objects spectroscopy,
spectropolarimetry, high-resolution photometry, etc) from different instruments
and space mission such as the VLTI, CHARA, FLAMES, ESPADONS-NARVAL, COROT,
MOST, SPITZER, etc, some discoveries were performed allowing to constrain the
modelling of the Be stars stellar evolution but also their circumstellar
decretion disks. In particular, the confrontation between theory and
observations about the effects of the stellar formation and evolution on the
main sequence, the metallicity, the magnetic fields, the stellar pulsations,
the rotational velocity, and the binarity (including the X-rays binaries) on
the Be phenomenon appearance is discussed. The disks observations and the
efforts made on their modelling is mentioned. As the life of a star does not
finish at the end of the main sequence, we also mention their stellar evolution
post main sequence including the gamma-ray bursts. Finally, the different new
results and remaining questions about the main physical properties of the Be
stars are summarized and possible ways of investigations proposed. The recent
and future facilities (XSHOOTER, ALMA, E-ELT, TMT, GMT, JWST, GAIA, etc) and
their instruments that may help to improve the knowledge of Be stars are also
briefly introduced.
