A number of organic materials have been observed to possess very large intrinsic nonlinear optical susceptibilities. Second and third order susceptibilities have been measured to be larger than any inorganic materials. These large susceptibilities as well as potentially very fast switching times arise from certain charge correlated features of the electronic states of the constituent molecular or polymeric units. We will review the research in this area, including basic studies into the origin of the susceptibilities on a molecular level. We will also discuss the requirements and properties of bulk materials including molecular and polymeric crystals, as well as thin films. Finally, we will discuss recent efforts to optimize microscopic and bulk properties aimed at making organic nonlinear optical materials a viable technology.