In this thesis, several new concepts for Raman amplifiers and lasers both in optical fibers and silicon waveguides have been developed, modeled and optimized that have the potential of increasing the performance of fiber-optic communication systems. Designs for Raman fiber lasers with improved efficiency, tunability, and output noise are analyzed. On the other hand, fundamental properties and limitations of silicon-based Raman amplifiers and lasers are investigated. Based on a newly developed comprehensive mathematical model of nonlinearly coupled wave propagation in silicon waveguides, new designs with increased efficiency are proposed.