This chapter examines the linear viscoelastic properties of flexible polymers, both in dilute solution and in the molten state. The J. C. Maxwell and W. Voigt models provide useful insight into the nature of viscoelastic response, but are severely lacking in terms of providing a satisfying description of real polymer liquids. The real polymer segments will move freely within the tube due to the rapid segmental motions, but the whole chain will only escape the tube by reptating out of the ends. The mechanical strength of a polymer part will depend on achieving full interpenetration or entanglement of the various molecules. In a molten polymer, the difference between the timescale for monomer motion and the timescale for the entire chain motion can be much greater than in dilute solution. In the particular case of a molten flexible polymer, the gap in the relaxation time spectrum arises for a purely topological reason, referred to as intermolecular entanglements.