  Recent observational constraints restrict the strict applicability of stellar
dynamics in spirals to a few rotation periods. However, stellar dynamics
concepts such as periodic orbits are invaluable for understanding the various
dynamical processes occurring during much more periods. A distinction of two
instability types in stellar systems is pointed out, the first one being well
illustrated by the bar instability, and the second one by the bar bending
instability. In bars the third dimension brings essential dynamical effects
which modify the views about the history of bulges and the spiral secular
evolution. Bars may grow, bend, thicken, and dissolve into spheroidal bulges,
and spirals may evolve along the Hubble sequence in the sense Sd$\to$Sa. This
leads to a much more dynamical picture of isolated galaxies than imagined
before.
