A new interpretation ofMc, the critical molar mass for chain entanglement in polymers, is presented from the viewpoint of chain interpenetration in the melt. It is shown that, atM=Mc, the number of chains in the melt connected by chain interpenetration reaches approximately 10 being independent of chemical structure of the chain. This work also interprets the empirical rule of van Krevelen. Arguments are presented for the theory that the majority of the chain entanglements are not topological in nature, but rather a local interaction of cohesive nature.