"Molecular rheology" is the missing link between the macromolecular structure of polymeric materials and their viscoelastic properties in the melt state. It complements the engineering or continuum mechanics aspects of rheology, which generally ignores the molecular details of the objects under study. The pioneering vision of the diffusion and relaxation processes of flexible macromolecular chains initiated by P.-G. de Gennes has lead to very effective and predictive models of viscoelasticity of polymer melts, which go far beyond academic interest. We present, in this paper, two very different examples of application of molecular rheology: molecular design of block copolymers corresponding to expected end-user properties (which are directly linked to linear viscoelastic properties) and formulation of blends of linear polymers in order to get strain-hardening effects in non-linear viscoelasticity usually obtained with long-chain branched (LCB) materials. (c) 2005 Elsevier B.V. All rights reserved.