  This article discusses the description of wall-bounded turbulence as a
deterministic high-dimensional dynamical system of interacting coherent
structures, defined as eddies with enough internal dynamics to behave
relatively autonomously from any remaining incoherent part of the flow. The
guiding principle is that randomness is not a property, but a methodological
choice of what to ignore in the flow, and that a complete understanding of
turbulence, including the possibility of control, requires that it be kept to a
minimum. After briefly reviewing the underlying low-order statistics of flows
at moderate Reynolds numbers, the article examines what two-point statistics
imply for the decomposition of the flow into individual eddies. Intense eddies
are examined next, including their temporal evolution, and shown to satisfy
many of the properties required for coherence. In particular, it is shown that
coherent structures larger than the Corrsin scale are a natural consequence of
the shear. In wall-bounded turbulence, they can be classified into coherent
dispersive waves and transient bursts. The former are found in the viscous
layer near the wall and as very-large structures spanning the boundary layer
thickness. Although they are shear-driven, these waves have enough internal
structure to maintain a uniform advection velocity. Conversely, bursts exist at
all scales, are characteristic of the logarithmic layer, and interact almost
linearly with the shear. While the waves require a wall to determine their
length scale, the bursts are essentially independent from it. The article
concludes with a brief review of our present theoretical understanding of
turbulent structures, and with a list of open problems and future perspectives.
