  The development of a coherent conceptual basis for the treatment of
non-linear microscopic phenomena, such as, hydrodynamic interaction, finite
extensibility, excluded volume and internal viscosity, in molecular theories of
dilute polymer solutions, is discussed. In particular, recent advances in the
treatment of hydrodynamic interaction are reviewed, and the successive
refinements which have ultimately led to the prediction of universal
viscometric functions in theta solvents are highlighted.
