This article explores the dynamics of entangled polymers, with particular emphasis on how the unusual and often dramatic mechanical properties of concentrated polymer systems are determined by the physics of entanglements. It begins with an overview of the foundations of entangled polymer dynamics, organized around tubes and slip links used in modeling entanglements, the packing length and concentration effects, the results of computer simulations on entanglements, topological contacts, and the effects of large deformations. The focus is on the nature of ‘entanglement’, both from a bottom-up molecular view, and from a phenomenological one. The discussion then turns to the linear viscoelasticity of entangled polymer solutions and melts, along with nonlinear viscoelasticity. Models of polymer dynamics in the linear regime are also described, including the ‘standard tube model’. The article concludes with suggestions for future work.