It is now clear that central star binarity plays a key role in the formation and evolution of planetary nebulae, with a significant fraction playing host to close-binary central stars which have survived one or more common envelope episodes. Recent studies of these systems have revealed many surprises which place important constraints on the common envelope - a critical phase in the formation of a wide variety of astrophysical phenomena, including the cosmologically important supernovae type Ia and other transient phenomena which will be detected by next-generation facilities, like the Large Synoptic Survey Telescope and the space-based gravitational wave detector the Laser Interferometer Space Antenna.