  Binarity and mass transfer appear to play a key role in the shaping and,
possibly, formation of planetary nebulae (PNe), thereby explaining the large
fraction of axisymmetric morphologies. I present the binary hypothesis for PNe
and its current status. Recent discoveries have led to a quadrupling of the
number of post-common envelope binary central stars of PNe compared to the end
of the last century, thereby allowing us to envisage statistical studies.
Moreover, these binary systems let us study in detail the mass transfer
episodes before and after the common envelope, and I present the evidences for
mass transfer - and accretion - prior to the common envelope phase.
