  Planetary nebulae (PNe) are circumstellar gas ejected during an intense
mass-losing phase in the the lives of asymptotic giant branch stars. PNe have a
stunning variety of shapes, most of which are not spherically symmetric. The
debate over what makes and shapes the circumstellar gas of these evolved,
intermediate mass stars has raged for two decades. Today the community is
reaching a consensus that single stars cannot trivially manufacture PNe and
impart to them non spherical shapes and that a binary companion, possibly even
a sub-stellar one, might be needed in a majority of cases. This theoretical
conjecture has however not been tested observationally. In this review we
discuss the problem both from the theoretical and observational standpoints,
explaining the obstacles that stand in the way of a clean observational test
and ways to ameliorate the situation. We also discuss indirect tests of this
hypothesis and its implications for stellar and galactic astrophysics.
