This thesis contains investigations of stimulated Raman scattering (SRS) in high-power continuous wave fiber lasers with diffraction-limited beam quality. A high-power, narrow-linewidth Raman fiber amplifier is constructed and SRS-induced transverse mode instability (TMI) is experimentally observed in a passive fiber. Influential parameters (like the fiber length) on the TMI threshold are investigated. Additionally, the spectral linewidth of the Raman fiber amplifier is investigated. Furthermore, walk-off between the Stokes signal and the Raman pump signal was found to have a major influence on the amount of Stokes light created in high-power fiber oscillators. An analytical model quantifying this effect for high-power cw fiber lasers is developed.