Absctract: Galactic internal structures are an important tracer of global-galaxy formation and evolution. Spatially resolved stellar populations of galactic disks tell us the story of spiral and lenticular galaxies, from the early stages when the older thick disks formed, to recent times when most thin disks were still forming stars. Thick disks, fainter envelopes of the brighter thin disks, are generally older and more metal-poor. Different formation scenarios have been proposed in the literature to explain their properties: in-situ formation at high redshift from turbulent gas, dynamical heating of a preexisting thinner disk and direct accretion of stars. I will present my work on this topic, combining high-quality integral-field spectroscopy observations of edge-on galaxies with high-resolution numerical simulations. Complex stellar populations suggest the presence of an extragalactic contribution and reveal that thick disks result from the combination of different formation mechanisms. While this complexity was first shown in star-formation histories from spectroscopic studies, simulations reveal one other piece of the puzzle: the in-situ or ex-situ origin of different stellar populations. Thick- and thin-disk star-formation histories and chemical evolution result from the interplay between internal and external processes.