| Summary: | I summarize the dynamical mechanisms that have been found to shape structures such as the spirals and the bars in barred-spiral systems. Response models and orbital analysis are used to study the dynamics in potentials estimated from near-infrared images of galaxies and are considered therefore as realistic. Contrarily to what is found in normal (non-barred) spirals, where the spiral arms are supported by regular orbits trapped around stable periodic orbits, in barred spiral systems the spirals are reinforced by particles in chaotic motion. The standard mechanism is associated with the unstable Lagrangian points close to the ends of the bar. Then the principal, “chaotic”, spirals extend beyond corotation. However, there are also cases where the chaotic orbits that support the spirals stay always inside corotation. Particles in chaotic motion are found to reinforce also the outer envelopes of the bars.
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