The explanation of the in-plane tensile stress-strain curve of paper has long been a matter for debate. In an earlier study it was shown that the elastic modulus of paper is given by an equation Ep = aφEf, where a is a function of the orientation distribution of the fibres in the sheet, φ describes the efficiency of stress transfer between them, and Ef is the elastic modulus of the fibres. As a result of extensive work on the effect of various paper-making treatments on the stress-strain response of paper, we have now shown that the plastic regime can be described in a similar manner, that is to say, in terms of the visco-elastic properties of the fibres, the orientation factor, and the efficiency factor. It is concluded that the non-linear behaviour of the stress-strain curve of paper originates primarily from the properties of the component fibres and not from the sheet structure.