Recent development on Raman amplifiers for WDM communication is reviewed. The design and demonstration of Raman amplifiers are summarized with concentration on Raman gain equalization by using multi-wavelength pumping scheme. Polarization dependence of Raman gain is measured against the degree of polarization of pump source. In order to realize stable and low degree of polarization, it is investigated how to design a passive depolarizer for pump laser diodes. Finally, a grating-integrated pump laser diode is developed in order to reduce relative intensity noise, which can cause severe degradation of amplifier characteristics.