S.Chmutov, S.Duzhin. The Kontsevich integral. This is an expository paper whose purpose is to explain the famous contribution of Maxim Kontsevich to Vassiliev knot invariant theory. More specifically, our aim is to present a proof of Kontsevich's fundamental theorem, which states that for each weight system there exists a Vassiliev invariant whose symbol is exactly the given weight system, or, in other words, that the only relations in the graded algebra of Vassiliev knot invariants are the 1- and 4-term relations. We also discuss the behaviour of the Kontsevich integral with respect to mirror reflection, change of orientation and mutations.