In conventional DFB semiconductor lasers, the distributed feedback of optical wave is provided by the periodic perturbation of waveguide index along the laser axis. This index- coupled DFB laser has a troublesome problem of producing a pair of longitudinal modes having equal threshold gain. Kogelnik and Shank predicted that this threshold degeneracy could be removed if the optical feedback was provided by the periodic perturbation of gain coefficient. The attempt to apply the gain coupling mechanism to DFB semiconductor lasers was started in 1988 by the present authors. In these four years it has been elucidated that the gain-coupled DFB semiconductor lasers have many advantages such as excellent single mode nature, reflection insensitivity, and low-chirping short optical pulse generation capability. In this talk various kinds of the gain-coupled DFB lasers developed so far and their main features are introduced and reviewed.