  I list the 16 planetary nebulae (PNe) known to contain close-binary nuclei,
and show that the nebulae generally have axisymmetric structures, including
elliptical, bipolar, or ring morphologies. The orbital periods range from 2.7
hr to 16 days, and close binaries constitute ~10% of all central stars. Since
the known binaries were found mainly from photometric variability, which
depends on heating effects at very small stellar separations, radial-velocity
surveys will be necessary to find the large predicted population of binary
nuclei with periods of about 10-100 days. Other PN phenomena that may arise
from binary-star interactions include jets and point-symmetry, the periodically
spaced arcs revealed by HST in the faint halos around several PNe and
proto-PNe, and the existence of PNe in globular clusters. There is thus
considerable circumstantial evidence that binary-star processes play a major
role in the formation and shaping of many or even most PNe.
