  A live halo plays an active role in the formation and evolution of bars by
participating in the angular momentum redistribution which drives the dynamical
evolution. Angular momentum is emitted mainly by near-resonant material in the
bar region and is absorbed mainly by near-resonant material in the halo and in
the outer disc. This exchange determines the strength of the bar, the decrease
of its pattern speed, as well as its morphology. Thus, contrary to previous
beliefs, a halo can help the bar grow, so that bars growing in galaxies with
responsive massive haloes can become stronger than bars growing in disc
dominated galaxies. During the evolution the halo does not stay axisymmetric.
It forms a bar which is shorter and fatter than the disc bar and stays so all
through the simulation, although its length grows considerably with time. I
discuss the orbital structure in the disc and the halo and compare it with
periodic orbits in analytical barred galaxy potentials. A central mass
concentration (e.g. a central black hole, or a central disc) weakens a bar and
increases its pattern speed. The effect of the central mass concentration
depends strongly on the model, being less strong in models with a massive
concentrated halo and a strong bar.
