Metamaterials are materials whose optical properties can be designed through the accurate engineering of their structure on the subwavelength scale. They have enabled the discovery and study of a variety of interesting new optical properties not normally present in materials found in nature. Furthermore, by designing the local electromagnetic field distributions of such metamaterials, it is possible to engineer not only their linear optical properties but also their nonlinear response, which is fundamental for the development of nonlinear and active nanophotonics for all-optical information processing. In this thesis I will show that plasmonic metamaterials based on metallic nanorod arrays can be designed to have strong third-order nonlinear optical response originating from the nonlinearity of the plasmonic component of the metamaterial, allowing nonlinear processes to be more energy efficient and highly integrated. The nonlinearity will be experimentally determined through the z-scan technique and explained by numerical modeling in both effective medium and fullvectorial simulations. Enhancements of about 50 times for the nonlinear absorption and about 10 times for the nonlinear refraction are observed compared to a smooth metal film. Furthermore, the properties of waveguides comprised of the nanorod metamaterial are studied and the possibility of their integration in conventional Si photonic waveguides is demonstrated. In this context, two all-optical modulators using plasmonic metamaterials are designed, operating in the hyperbolic and epsilon near-zero regimes. Both designs are highly integrated and energy efficient having footprints of 300x440x600 nm3 and 300x180x340 nm3 with an energy consumption of 3.7 pJ/bit and 0.6pJ/bit respectively. The obtained results show great opportunities for nonlinear metamaterials in nanophotonic applications.