The macroscopic behavior of the contact Line during the spontaneous spreading of a droplet can be described in terms of microscopic quantities, specifically the substrate-liquid and Liquid-liquid interactions. Here, for the first time, the results of molecular dynamics simulations of very large sessile drop systems are compared with the predictions of the molecular-kinetic theory of wetting. Good agreement is obtained, with both approaches yielding a consistent set of molecular parameters and macroscopic behavior which is consistent with experiments.