Important theoretical results have been obtained in the fifties, sixties, (KOC theory, Batchelor regime,.. ) In the last ten years, theory has made important progress for the case of rough velocity fields essentially after the Kraichnan model (1968) was rigorously solved (in 1995 by two groups) In the meantime, the case of smooth velocity fields, called the Batchelor regime, has been solved analytically.