  Wall-bounded flows experience a transition to turbulence characterized by the
coexistence of laminar and turbulent domains in some range of Reynolds number
R, the natural control parameter. This transitional regime takes place between
an upper threshold Rt above which turbulence is uniform (featureless) and a
lower threshold Rg below which any form of turbulence decays, possibly at the
end of overlong chaotic transients. The most emblematic cases of flow along
flat plates transiting to/from turbulence according to this scenario are
reviewed. The coexistence is generally in the form of bands, alternatively
laminar and turbulent, and oriented obliquely with respect to the general flow
direction. The final decay of the bands at Rg points to the relevance of
directed percolation and criticality in the sense of statistical-physics phase
transitions. The nature of the transition at Rt where bands form is still
somewhat mysterious and does not easily fit the scheme holding for
pattern-forming instabilities at increasing control parameter on a laminar
background. In contrast, the bands arise at Rt out of a uniform turbulent
background at a decreasing control parameter. Ingredients of a possible theory
of laminar-turbulent patterning are discussed.
