  Angular momentum redistribution within barred galaxies drives their dynamical
evolution. Angular momentum is emitted mainly by near-resonant material in the
bar region and absorbed by resonant material mainly in the outer disc and in
the halo. This exchange determines the strength of the bar, the decrease of its
pattern speed, as well as its morphology. If the galaxy has also a gaseous
component and/or a companion or satellite, then these also take part in the
angular momentum exchange. During the evolution a bar structure forms in the
inner parts of the halo as well. This bar is shorter and fatter than the disc
bar and stays so all through the simulation, although its length grows
considerably with time. Viewed edge-on, the bar in the disc component acquires
a boxy or peanut shape. I describe the families of periodic orbits that explain
such structures and review the observations showing that boxy/peanut `bulges'
are in fact just bars seen edge-on.
