  Having once thought that we understood how some galaxies were barred and had
difficulty accounting for the absence of bars in others, it now seems that we
have the opposite problem. Most real galaxies have centres dense enough to
inhibit bar formation, even if they have massive discs. This is also true for
barred galaxies, which therefore could not have acquired their bars as a result
of a self-excited, global instability. There is a hint from the morphology of
galaxies in the Hubble Deep Fields that the growth of bars might be a slow or
secular process. Here I discuss possible mechanisms that could form bars long
after the disc is assembled.
