Much of current astrophysical research is aimed at addressing one key issue:
how galaxies form and evolve. We still do not fully understand the
evolutionary processes driving the lifetimes of the zoo of galaxies which populate the Universe.
Galaxies may be isolated, or in groups or clusters; they may appear as pure ellipsoids or discs or
include a variety of structures; they may be forming stars violently or passing
quietly through their lives; and the many other properties that we are progressively discovering.
Each piece we add to the puzzle pile complicates the picture a bit more. This thesis
is aimed at fixing one of these pieces, a specific one related to the very interesting, but
so far not well characterised, double-barred galaxies.

Barred galaxies are rather common structures in the Universe and, more importantly,
they are key elements for secular evolution theories. In fact, bars can
transport gas to the central regions of galaxies and trigger the formation of bulges and other
new structures. Double bars go a step farther: they allow the material to reach the very central regions
where the gas driven inwards by a single bar cannot get to. Therefore, nested bar systems are considered
a very efficient way to bring about the internal secular evolution of galaxies and even to feed
active galactic nuclei. This hypothesis has, however, some detractors, as it is very promising
from a theoretical point of view but there is not much observational evidence that it is valid.

In this thesis we have carefully observed and studied a sample of five double-barred galaxies using
the state-of-the-art techniques for the analysis of their kinematics and stellar populations.
This has led to original work not previously performed, so each result is relevant and provides
clues to the nature of these objects. We are pleased to present the
discovery of the $\sigma$-hollows, the only known kinematical signature of the presence of stellar inner bars.
Moreover, we disentangle the complex structural composition of the galaxies of our sample, dealing
with different formation scenarios and rejecting those which do not account for the observational
properties derived here. We find out that one out of the five galaxies is probably
hosting a disc-like bulge, whereas the other four present a classical bulge in their centres.
Finally, we ascertain that inner bars tend to be younger and more metal-rich than the outer structures.
Nevertheless, the characteristic stellar populations for the bulge and the inner bar are
indistinguishable and there is no evidence of star-forming structures in the central
regions of these galaxies, so we have to conclude that these five inner bars are not playing a major role
in the secular evolution of their host galaxies.