Turbulence is a dangerous topic which is often at the origin of serious
fights in the scientific meetings devoted to it since it represents
extremely different points of view, all of which have in common their
complexity, as well as an inability to solve the problem. It is even
difficult to agree on what exactly is the problem to be solved.
Extremely schematically, two opposing points of view have been advocated
during these last ten years: the first one is "statistical", and tries
to model the evolution of averaged quantities of the flow. This com- has
followed the glorious trail of Taylor and Kolmogorov, munity, which
believes in the phenomenology of cascades, and strongly disputes the
possibility of any coherence or order associated to turbulence. On the
other bank of the river stands the "coherence among chaos" community,
which considers turbulence from a purely deterministic po- int of view,
by studying either the behaviour of dynamical systems, or the stability
of flows in various situations. To this community are also associated
the experimentalists who seek to identify coherent structures in shear
flows.