In this paper it is shown that three critical concentrations, observable through light scattering, solution and melt viscosity, exist for high molecular weight polymers: Ci, observed through light scattering, reflects the onset of intermolecular interpenetration at the point of complete space-filling, the point where all the available space is taken up by segments with no change in the intramolecular segmental density; Cep, observed through solution viscosity determination, reflects the appearance of dynamically measurable quantities of entanglements in the peripheral regio of the molecular coils; and Cec, obtained from melt viscosity measurements, reveals the appearance of such entanglements in the core regions of the coils. The relationship between these critical concentrations is Ci< Cep< Cec. This reflects the fact that intermolecular interpenetration occurs at significantly lower concentrations than intermolecular entanglements, and that the static light scattering is insensitive to entanglements while the dynamic viscosity measurements are sensitive to entanglements but do not sense intermolecular interpenetrations. A tentative explanation of these observations, based on the macromolecular domain model, is proposed.