Distributed Fiber Raman Amplifier (DFRA) is a key component for future long-distance fiber communication systems since it uses the transmission fiber link as the amplifying (active) medium. However, DFRAs must be designed with a wide flat-gain profile if they are used to amplify wavelength division multiplexing (WDM) signals. In this thesis, the performance of DFRAs is characterized for WDM systems using a comprehensive theoretical model. The model describes the variation of wave's power along the fiber when the DFRA is operating with multisignals and multipumps. The model takes into account pump-to-pump, signal-to-signal, and pump-to-signal interactions as well as the fiber loss. The model is also applicable for co-propagating, counter-propagating, and bidirectional pumping configurations. Simulation results are presented to characterize the DFRA operating with different multipumping configuration schemes. The results can be used as a guideline to design a DFRA with a flat-gain profile for WDM systems. The results indicate that fifty WDM signals having 1.6 nm channel separation can be amplified with a 70 km single mode fiber (SMF) pumped by five pumps. A gain ripple of 1.4 dB is achieved over 80 nm band with total pumping power less than 800 mW. The simulation results presented in this thesis are performed using MATLAB 7 software. For comparison purposes, some of the results are carried out using Optisystem4.0 software. The results obtained by both software packages are in good agreement.