Fibrous materials such as paper, board, and bio-based composites have complex 3-D internal structures. X-ray computed tomography (XRCT) at the micron and sub-micron scale is a viable technique for visualizing and characterizing the internal three dimensional structures of fibrous materials. XRCT can be used as a non-invasive method to determine the pore structure characteristics including porosity, fiber-pore interfacial surface area, pore size distribution and tortuosity in three dimensions. Comparison of the 3D pore size distributions between handsheets prepared under different conditions showed that as the degree of mechanical refining of fibers is increased, the pore sizes decrease along with a narrowing of the distributions. The effects of fiber species and papermaking process variables such as wet-pressing on the 3D internal structure are also shown. Comparison with conventional techniques shows the advantages of the XRCT based structural characterization. In addition, methods to determine the vapor, liquid transport and mechanical properties of paper and board using actual 3D structures show the versatility and wide ranging applications of this technique. While structure plays a significant role in affecting moisture transport resulting in anisotropic behavior, interestingly, above a certain value for fiber space conductivity, porous materials, which are initially anisotropic, behave more like an isotropic material.