Interfacial energy plays the role of a multiplicative prefactor to rate-dependent rheological phenomena in practical measurements of adhesion energy. Interfacial energy can also be correlated, in ways yet to be fully understood, with measurements of coefficient of friction between solid surfaces. In this note, some points are made about the utility of the JKR methods of contact mechanics for the accurate determination of interfacial energies. Issues that arise are : the adaptability of the methods to various types and states of polymeric materials; rearrangement of surfaces during interfacial contact and; viscoelasticity of materials and its influence on JKR interfacial energy measurement.