A viewpoint on the importance of the accurate determination of the crystal modulus of cellulose is given with respect to recent advances in extracting high modulus nanofibrils/fibers from plants and production by bacteria and animals. Often the modulus of these nanofibrils/fibers is assumed to be the same as the crystal modulus of cellulose. Both experimental and theoretical calculations of the crystal modulus of cellulose are discussed and put in context of the use of cellulose nanofibers for high stiffness composites. New research into the exact nature of the structure of cellulose nanofibers is suggested, with a view to better processing routes for realizing high modulus/low density materials. © 2012 American Chemical Society.