Planetary nebulae are traditionally considered to represent the final
evolutionary stage of all intermediate-mass stars (∼0.7-8
M⊙). Recent evidence seems to contradict this picture.
In particular, since the launch of the Hubble Space Telescope, it has
been clear that planetary nebulae display a wide range of striking
morphologies that cannot be understood in a single-star scenario,
instead pointing towards binary evolution in a majority of systems.
Here, we summarize our current understanding of the importance of
binarity in the formation and shaping of planetary nebulae, as well as
the surprises that recent observational studies have revealed with
respect to our understanding of binary evolution in general. These
advances have critical implications for the understanding of mass
transfer processes in binary stars—particularly the all-important
but ever-so-poorly understood 'common envelope phase'—as well as
the formation of cosmologically important type Ia supernovae.