Gas-filled hollow-core fiber lasers (HCFLs) not only combine the advantages of fiber lasers and gas lasers but also provide an efficient way to develop high-energy fiber lasers in the mid-infrared (IR) spectral range. The development of new designs and manufacturing technology for silica glass hollow-core fibers was key to success in obtaining laser generation at new wavelengths in the mid-IR spectral range. To date, gas-filled HCFLs based on stimulated Raman scattering and population inversion between rotational and vibrational energy levels of the gases filling the air-core have been demonstrated. This chapter reviews major aspects of these topical issues and outlines new promising approaches emerging in the field of gas-filled HCFLs.