  We discuss different aspects of nested bar systems, both observational and
theoretical. Such systems consist of a large-scale primary bar leading to the
formation of a sub-kpc size secondary bar, whose pattern speed differs
substantially from that of the main bar. Specifically, we focus on the origin
and gravitational decoupling of nested bars, and their characteristic gas flows
on scales of about 100 pc -- 10 kpc, with and without gas gravity. We find that
the gas response in nested bars differs profoundly from that in single bars,
and that no offset dust lanes form in the secondary bars. We also discuss
briefly the importance of nested bar systems for redshifts corresponding to
galaxy formation epoch.

Astrophysics