This review has highlighted some recent work which further illuminates the tight physical links between bars, nuclear rings, and their host galaxies. These relationships are shedding light on the underlying dynamical structure, and on the overall process of secular evolution of galaxies. The dynamics of bars is now rather well understood. They indeed stimulate inflow of gaseous material from the disk to the central region of a galaxy, and thus inflowing gas can help build the bulge and drive evolution, even across morphological types. Nuclear rings are common, and transform significant amounts of gas into stars, again increasing the bulge mass. Nuclear rings occur mostly in barred galaxies, but the fraction occurring in unbarred galaxies is similar to the overall fraction of unbarred disk galaxies, so a causal link is not proven. Ovals, interaction event, or possibly even strong spiral arms can cause enough non-axisymmetry in the gravitational potential to stimulate the formation of a nuclear ring, even in the absence of a bar. This shows how ubiquitously secular evolution in galaxies can occur.
The future for this area of research is bright, because significant amounts of new data are being collected which will allow us to increase sample sizes and progress to study secular evolution on a more fundamental basis. We explicitly mentioned the S4G survey, which will deliver deep, mid-IR imaging covering the complete disks of more than 2300 galaxies in the local Universe.
Acknowledgements I thank my co-workers Ron Buta, Sébastien Comerón, Eija Laurikainen, and Heikki Salo for comments on an earlier draft of this paper. I thank them and my other collaborators, including those on the S4G team, for stimulating discussions and scientific progress.