From a variational principle, we derive the equations of motion for nematic liquid crystals described by a second-rank order tensor. The only constitutive ingredients are the densities of the free energy and the dissipation, both subject to appropriate invariance requirements. We show that this theory is sufficiently general to encompass others as special cases. Comparing this theory with other theories also suggests criteria to avoid the proliferation of viscosity coefficients. (C) 2003 Elsevier B.V. All rights reserved.