Are all planetary nebulae produced by binary interactions?
It is likely that many more short period binaries reside in the middle of planetary nebulae (PN) than is currently known. In fact, PN formation and shaping theories recently shifted to predicting that most if not all PNe derive from binary interactions, in particular common envelopes (CE). I will describe population synthesis work that agrees with this hypothesis as well as on-going observational tests which are being carried out. In addition, post-CE central star binaries are the start point for the evolution of cataclysmic variables, and are fundamental to understand the presence, numbers and characteristics of systems such as novae, dwarf novae and, possibly, the progenitors of Type Ia supernovae. For this reason we are carrying out hydrodynamic CE simulations to determine the type of systems that can emerge from this interaction and the final periods. Finally I will discuss the difficulties of carrying out CE simulations, and take a devil's advocate position on how realistic the current generation of results can actually be.