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Identification of a sample of extended-UV (XUV) disks within the S4G sample by the means of IRAC and GALEX data. Roughly 450 type-1 and 250 type-2 extended-UV disks are expected. Investigate the past star formation history of the XUV disks, using an analysis of the IRAC surface brightness profiles in both XUV and non-XUV disk galaxies. Study of the truncation of stellar disks in both samples. Chemical evolution of the outer disks is a further clue to the study of the early evolution of disks, gas infall, and upper-end IMF properties, and will be studied with optical spectroscopy of the HII regions in XUV disks. Derivation of specific star formation rate profiles using IRAC and GALEX data, and comparison with models for the evolution of disks with and without stellar migration. Comparing with HI data, study disk growth and the star formation law in outer disks.