  Bars play a major role in driving the evolution of disk galaxies and in
shaping their present properties. They cause angular momentum to be
redistributed within the galaxy, emitted mainly from (near-)resonant material
at the inner Lindblad resonance of the bar, and absorbed mainly by
(near-)resonant material in the spheroid (i.e., the halo and, whenever
relevant, the bulge) and in the outer disk. Spheroids delay and slow down the
initial growth of the bar they host, but, at the later stages of the evolution,
they strengthen the bar by absorbing angular momentum. Increased velocity
dispersion in the (near-)resonant regions delays bar formation and leads to
less strong bars. When bars form they are vertically thin, but soon their inner
parts puff up and form what is commonly known as the boxy/peanut bulge. This
gives a complex and interesting shape to the bar which explains a number of
observations and also argues that the COBE/DIRBE bar and the Long bar in our
Galaxy are, respectively, the thin and the thick part of a single bar. The
value of the bar pattern speed may be set by optimising the balance between
emitters and absorbers, so that a maximum amount of angular momentum is
redistributed. As they evolve, bars grow stronger and rotate slower. Bars also
redistribute matter within the galaxy, create a disky bulge (pseudo-bulge),
increase the disk scale-length and extent and drive substructures such as
spirals and rings. They also affect the shape of the inner part of the
spheroid, which can evolve from spherical to triaxial.
