The strength and stiffness properties of paper show a high degree of variation, since cellulosic fibers are highly heterogeneous on the microstructural level. These properties are mainly influenced by the strenght properties of single fibers and of the fiber-fiber joints in the fiber network. Futher the size and number of the bonding areas between the fibers is very important. Because of the hygroscopic behaviour of the wood polymers cellulose and hemmicelluloses the mechanical properties are also depending on moisture and temperature. The high degree of heterogeneity on the microstructural level and the high number of number of loading modes of fibers and fiber-fiber joints in paper make it almost impossible to analyze this behaviour using exclusively experimental methods. The application of analytical and numerical methods from the field of mechanics to evaluate the structure-property relationships of paper at different length scales is therefore of high interest. In this project micro-tensile and -stiffness test on fibers and fiber-fiber joints are combined with numerical modelling in order to determine the important parameters under varying failure modes. By combining novel experimental methods and modelling a better understanding of the structural/dynamical behaviour of paper and its behaviour under load will be obtained.