  Almost fifty years of investigations of barchan dunes morphology and dynamics
is reviewed, with emphasis on the physical understanding of these objects. The
characteristics measured on the field (shape, size, velocity) and the physical
problems they rise are presented. Then, we review the dynamical mechanisms
explaining the formation and the propagation of dunes. In particular a complete
and original approach of the sand transport over a flat sand bed is proposed
and discussed. We conclude on open problems by outlining future research
directions.
