The literature concerning the structure of two- and three- dimensional fibre networks is reviewed. Emphasis is placed on the literature concerning such networks in papermaking processes, though examples are drawn from other systems. The propensity of a suspension to flocculate is considered from a theoretical viewpoint. The experimental techniques and structural descriptors applied in the characterisation of fibre networks are discussed. Theoretical studies of random networks are presented along with their use as reference structures and comparison is made between the main techniques used in the structural characterisation of essentially two-dimensional networks such as paper. The relationships between the distributions of mass and voids are examined and the dependence of sheet nonuniformity on that of the suspension from which it is formed is reviewed.