Transparent wood-polymer biocomposites (TWPBs) are interesting wood-based materials with a unique combination of optical transmittance and mechanical properties. The wood substrate is delignified (removal of lignin) while the wood microstructure is preserved. In this thesis, the deformation and fracture behavior of TWPBs are investigated, and material properties are determined. In addition, a method is presented for identifying orthotropic and fracture properties from single small and thin specimen geometries of wood composites by minimizing the discrepancy between experimentally measured and numerically generated strain fields. Material model parameters in the finite element method (FEM) are updated by optimization routines (FEMU). The focus of the thesis is on TWPBs, but wood fiber biocomposites are included to develop the FEMU approach. InPaper I and II, the mechanical and optical behavior of laminated and single lamina of TWPBs are investigated. Orthotropic mechanical properties, such as elastic stiffness parameters and tensile strength, are determined along and across the fiber direction, and the deformation mechanisms are characterized. Reducedanisotropic ratio (e.g., a ratio of in-plane elastic stiffness parameters), increased wood cell wall effective properties, and improved stress transfer by a more homogeneous strain field are found for TWPBs compared to native wood. Lamination moderates the weakest properties and allows structural tailoring, making it more suitable as a load-bearing material.In Paper III and IV, fracture and deformation mechanisms are investigated, and the fracture properties are determined using cohesive zone models (CZM) along and across the fiber direction. This approach made it possible to explain 90° crack deflection phenomenon. Large fracture process zones (FPZ) dominated by the cell wall properties are observed, involving fiber pull-out with large cohesive strength in the fiber direction. Also, cross-over bridging mechanisms by cell wall peeling in the transverse fiber direction, with low cohesive strength properties. Longitudinal fracture properties of native wood are improved with a polymer matrix, while the transverse fracture properties are reduced as well as the size of the FPZ.In Paper V, random and oriented wood-fiber biocomposites are investigated, relating nano- and microscale structures to macroscopic mechanical properties. Orthotropic elastic-plastic material parameters are identified from off-axis tensile tests by using FEMU. Fracture mechanisms are related to microstructural features by the use of supporting in situ tensile tests in a scanning electron microscope.