There is a wide class of materials where a fiber network is the main load-carrying component. This class encompasses paper, paperboard, tissue and many biopolymers. These materials are known to have outstanding stiffness-to-mass ratio which is one of the reasons they are so abandon in nature. We will study this class of materials by considering paper and its applications. Despite being simple in use, paper constitutes one of the most complex materials among those you encounter in your daily life. Paper can be easily tested mechanically and exhibits a whole range of behavior during converting and end-use. Paper is heterogeneous and anisotropic. In a number of applications, it is used beyond its elastic limit. The strength of paper is size-dependent and is different in tension and compression. Furthermore, paper absorbs water, which changes its mechanical properties and dimensions.