The detailed study of the different structural components of nearby galaxies can supply vital information about the secular, or internal, evolution of these galaxies which they may have undergone since their formation. We highlight a series of new studies on the thick and thin disk components of galaxies, based on the analysis of mid-infrared images of a sub-sample of the over 2000 local galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G). In particular, we show that the thick and thin disk components are roughly equally massive, and that their properties indicate that the thick disks mostly formed in situ, and to a much lesser degree as a result of galaxy-galaxy interactions. The occurrence of truncations and anti-truncations, studied separately in the thin and thick disk components, yields further constraints on the relative thin and thick disk masses, and on the evolutionary paths of their host galaxies.