Evidence is given showing that the equation, log&eegr;=3.4 logZ+K, represents an empirical law of flow which holds generally for sufficiently long flexible chain molecules in bulk or in solution. Here &eegr; is the viscosity,Zis the number of atoms in the chain, andKis a constant which is dependent on the polymer type and on the temperature. All of the available data which cover wide molecular weight ranges for linear (and branched) molecules in bulk or in solution, including results on polyesters, polyamides, polystyrene, polyisobutylene, polydimethyl siloxane, and polymethyl methacrylate, support this conclusion. For chains having fewer than a critical number,Zc, of chain atoms (Zcbeing characteristic of the polymer species) the dependence of &eegr; onZis less severe but more complex. These results are in semiquantitative agreement with the recent theory of F. Bueche. On the basis of this theory, values for the concentration of chain entanglements in various polymers are obtained from the observed values ofZc.