  Isolated barred galaxies evolve by redistributing their internal angular
momentum, which is emitted mainly at the inner disc resonances and absorbed
mainly at the resonances in the outer disc and the halo. This causes the bar to
grow stronger and its pattern speed to decrease with time. A massive,
responsive halo enhances this process. I show correlations and trends between
the angular momentum absorbed by the halo and the bar strength, pattern speed
and morphology. It is thus possible to explain why some disc galaxies are
strongly barred, while others have no bar, or only a short bar or an oval. In
some cases, a bar is found also in the halo component. This ``halo bar'' is
triaxial, but more prolate-like, is shorter than the disc bar and rotates with
roughly the same pattern speed. I finally discuss whether bars can modify the
density cusps found in cosmological CDM simulations of dark matter haloes.
