The naturally grown material wood exhibits a rather complex mechanical behavior. This is mainly caused by the growth-induced orthotropy of the clear wood material, and further increased by knots and the resulting fiber deviations in their vicinities. A reliable prediction of the mechanical behavior of wooden boards and wood products by means of numerical simulation tools is not easy to accomplish. Nevertheless, for a better utilization and a more efficient use of this valuable building material, the demand for such prediction tools increases continuously. Motivated by this need, within this work, simulation tools for wooden boards were developed, enabling the consideration of realistic three-dimensional fiber deviations around knots and taking the complex mechanical behavior of clear wood into account. The resulting stress and strain fields within the elastic regime and the accuracy of the calculated fiber directions are validated by means of four-point bending tests, which are accompanied by full-field deformation measurements based on digital image correlation techniques. Furthermore, a criterion, which indicates the point of structural failure, was developed. In the process, the effective strength values are determined by examining the qualitative stress changes in predefined volumes around knots. This approach is based on the formation of failure zones predominantly caused by perpendicular-to-grain tension in the vicinity of knots, and is confirmed by comparisons to four-point bending and tensile tests of wooden boards with various cross-sectional dimensions. In a next step, the failure processes themselves were described and linked to the cell structure of wood, because the initiation and also the global direction of cracks on the macroscale are governed by structural features on the lower length scales. To depict this behavior accurately, all possible failure mechanisms for two repetitive units, representing late- and earlywood, respectively, are identified by an approach based on the extented Finite Element method. By using sampling techniques, a wide range of possible loading combinations were generated and applied to the two cell types. For all simulations, the obtained failure modes were evaluated, classified and global crack directions assigned. The following definition of two multisurface failure criteria with predefined global crack directions for the two cell types individually allows the implementation into subroutines of commercial Finite Element software. Through application of this approach by simulating tensile tests on the annual year ring scale its capability to reproduce essential failure mechanisms in wood correctly could be shown. Finally, intended for direct practical application, the evaluation of common and the introduction of new indicating properties (IPs) for the strength grading of wooden boards are presented. While commonly used IPs hardly consider the 3D position and orientation of knots within wooden boards, new scanning techniques would allow highly improved yields within the wood grading process. Thus, algorithms for the manual and also automatic 3D reconstruction of wooden boards are presented. Based on this new knot information, novel IPs were developed, which allow for the consideration of knots, resulting fiber deviations and the knot location, which plays an important role under bending load. A statistical evaluation of common and novel IPs shows that by using these additional information significant improvements can be reached.