Composite materials have seen increasing applications in fields which require severe design criteria since their mechanical properties can be tailored for the intended structural strength and stiffness. Despite their anisotropic nature that can introduce flexibility to structural design, their behavior generally exhibits nonlinear responses during mechanical tests and complicates the characterization of composites. This book presents a new framework to characterize the nonlinear behavior of anisotropic materials in an on-line manner. The new framework applies recursive estimation and a multi-linear model to characterize the nonlinear behavior of anisotropic materials on-line using full-field strains, which are capable of capturing the multi-axial information of anisotropic materials.