  It is now clear that a binary evolutionary pathway is responsible for a
significant fraction of all planetary nebulae, with some authors even going so
far as to claim that binarity may be a near requirement for the formation of an
observable nebula. In this chapter, we will discuss the theoretical and
observational support for the importance of binarity in the formation of
planetary nebulae, initially focussing on common envelope evolution but also
covering wider binaries. Furthermore, we will highlight the impact that these
results have on our understanding of other astrophysical phenomena, including
supernovae type Ia, chemically peculiar stars and circumbinary exoplanets.
Finally, we will present the latest results with regards to the relationship
between post-common-envelope central stars and the abundance discrepancy
problem in planetary nebulae, and what further clues this may hold in
forwarding our understanding of the common envelope phase itself.
