Nonlinear optics is playing a vital role in the development of devices for all optical computing, data storage, senor protection and biosensing. In order to build such devices, it is essential to develop robust nonlinear optical materials. While a variety of organic and inorganic materials and their composites have been developed, there still remain various practical issues, such as optical loss, narrow band and slow optical response, low laser damage threshold, poor mechanical strength and environmental degradability, which set limitations for the use of most of the materials in devices. Therefore, any advancement in the fields of nonlinear optics and photonics largely depends on the development of materials and the accurate understanding of their nonlinear optical properties. This book reports on the nonlinear optical characterization of new organic materials (chalcones and thiophene-based copolymers) and new nanomaterials(NiTi nanoalloy). It presents several important issues associated with size-dependent optical properties of metal nanomaterials and a note on carbon nanotubes and nanoporous materials. A qualitative analysis of the size-dependent properties is described.