The 3rd octave chatter in cold rolling mills is a major issue for the industrial production of thin sheets with very tight tolerances, like packaging steels for instance. In this context, an experimental and theoretical approach, completed in partnership by Arcelor and VAI, afforded a precise analysis and innovative results, in the following fields : global on-line tandem mill instrumentation and efficient signal treatments implementation ; clear differentiation between brummer (non divergent mode) and chatter (divergent mode) ; modelling of interactions between chatter and technology (analytical and F.E. approaches) ; advanced chatter monitoring and industrial implementation.