A unified picture is presented of the formation of bar structures in disk galaxies of various morphological types. In order to discuss bar formation in the context of galactic disk formation, a simple analytic model is constructed of the growth of galactic disks by infall of primordial gas from haloes and subsequent star formation in the disks. It is monitored during the course of disk growth whether or not the condition for spontaneous bar formation (i.e., bar instability) is fulfilled for the stellar disk component.