I will review the multiscale approach to modelling of entangled polymers, which includes molecular dynamics (MD), single chain stochastic models (slip-springs) and the tube model. After that I will concentrate on the link between many chain (MD) and single chain models.  I will report results from molecular dynamics simulations on stress relaxation and show the detailed comparison with slip-spring model. In the second part of the talk I will turn to the issue of microscopic  definition of entanglement in molecular dynamics. We propose to define entanglement as a long-lived contact between mean paths of the two chains. Using this definition, we present empirical evidence and statistical properties of such entanglements, and discuss the implications for the tube theory and the slip-spring model.