  Contrasting with free shear flows presenting velocity profiles with
inflection points which cascade to turbulence in a relatively mild way, wall
bounded flows are deprived of (inertial) instability modes at low Reynolds
numbers and become turbulent in a much wilder way, most often marked by the
coexistence of laminar and turbulent domains at intermediate Reynolds numbers,
well below the range where (viscous) instabilities can show up. There can even
be no unstable mode at all, as for plane Couette flow (pCf) or for Poiseuille
pipe flow (Ppf) that currently are the subject of intense research. Though the
mechanisms involved in the transition to turbulence in wall flows are now
better understood, statistical properties of the transition itself are yet
unsatisfactorily assessed. A review of the situation is given. An alternative
to the temporal theory of the transition to turbulence in terms of chaotic
transients in such globally subcritical flows is proposed, which invokes
spatio-temporal intermittence and the theory of first order (thermodynamic)
phase transitions.
