This chapter covers recent advances in fiber lasers with distributed feedback (DFB) based on either regular or random refractive index structures. Unlike conventional lasers with Fabry-Perot cavities consisting of two mirrors (fiber Bragg gratings in the case of fiber lasers) at both ends of a gain medium, distributed feedback involves long reflective structures that form effective cavities along the gain medium. Experiments with passive and active fibers, in which regular or random reflective structures are formed, including π-shifted gratings inscribed by femtosecond (fs) pulses, gratings with random amplitude and phase variations, or random arrays of multiple gratings, are reviewed. The results are preceded by a theoretical background on Bragg reflectors and Bragg gratings with π-shift. When the cavity feedback and the amplification of light are distributed along the active fiber, the coupled wave equations become nonlinear, for which some analytical solutions are given. On the basis of this background, a record short DFB laser based on π-shifted grating fs-inscribed in a highly doped erbium fiber, high-efficiency single-frequency ytterbium and Raman lasers based on the random grating and the random grating array, respectively, as well as extreme line narrowing (by 4 orders) in a hybrid configuration involving DFB laser and fs-inscribed artificial Rayleigh reflector are demonstrated. The results are followed by a discussion of further developments and potential applications of DFB fiber lasers.