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. This chapter discusses the theoretical and observational support for the importance of binarity in the formation of planetary nebulae, initially focusing on common envelope evolution but also covering wider binaries. Furthermore, the chapter highlights the impact that these results have on our understanding of other astrophysical phenomena, includingType Ia supernovae, chemically peculiar stars and circumbinary exoplanets. Finally,the latest results will be presented with regard 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.