This lecture delves into the study of viscoelastic behavior in polymer liquids, exploring the molecular weight dependence of melt viscosity and the predictions of various models. It covers the rheological properties of polymers, the Williams-Landel-Ferry equation, and the role of entanglements in the viscoelastic response. The presentation also discusses the reptation model, additional relaxation processes, and the implications for polymer dynamics. Emphasis is placed on understanding the complex interplay between molecular structure, chain dynamics, and material behavior.