All-atom molecular dynamics simulations with explicit water solvent were used to investigate the microstructure and conformational dynamics of cellulose Iβ microfibrils as a function of microfibril length and cross-sectional size and shape. Cellulose microfibrils quickly develop a right-handed twist, which then remains stable over the entire 10 ns simulation time. The helical angle is independent of microfibril length and inversely proportional to its cross-sectional area, in accord with the expectations of continuum theory for an intrinsic chiral twist that is opposed by torsional shear. These calculations provide-to our knowledge-the first estimates of the shear modulus of a cellulose microfibril from MD simulations. The internal strains caused by this helical twist, propagated indefinitely along the microfibril axis, could be relaxed by periodic regions of amorphous structure along the axis of the cellulose microfibrils.