Silicon-On-Insulator integrated optics boasts low loss waveguides and tight optical confinement necessary for the design of nanophotonic devices. In addition, the processing is fully compatible with capabilities of standard silicon foundries. Because of crystal symmetry, silicon does not possess 2nd order nonlinear optical effects. However, the combination of nanoscale geometries with the high refractive index contrast creates high optical intensities where 3rd order effects may become important, and in fact, may be exploited. In this context, we study the two main nonlinear processes that can occur in silicon waveguides, namely Stimulated Raman Scattering (SRS) from zone-center optical phonons and Two-Photon Absorption (TPA). Because of the single crystal structure, the Raman gain coefficient in silicon is several orders of magnitude larger than that in the (amorphous) glass fiber while its bandwidth is limited to approximately 100GHz. To achieve Raman gain in the 1550nm region requires the pump to be centered at around 1427nm. We discuss the Raman selection rules in a silicon waveguide and present the design of an SOI Raman amplifier. We show that by causing pump depletion, TPA can limit the amount of achievable Raman gain. TPA also limits the maximum optical SNR of the silicon amplifier.