Binary stars as the key to understanding planetary nebulae

Planetary nebulae are traditionally considered to represent the final evolutionary stage of all intermediate-mass stars (similar to 0.7-8 M circle dot). 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.