  Nanofluidics has emerged recently in the footsteps of microfluidics,
following the quest of scale reduction inherent to nanotechnologies. By
definition, nanofluidics explores transport phenomena of fluids at the
nanometer scales. Why is the nanometer scale specific ? What fluid properties
are probed at nanometric scales ? In other words, why 'nanofluidics' deserves
its own brand name ? In this critical review, we will explore the vast manifold
of length scales emerging for the fluid behavior at the nanoscales, as well as
the associated mechanisms and corresponding applications. We will in particular
explore the interplay between bulk and interface phenomena. The limit of
validity of the continuum approaches will be discussed, as well as the numerous
surface induced effects occuring at these scales, from hydrodynamic slippage to
the various electro-kinetic phenomena originating from the couplings between
hydrodynamics and electrostatics. An enlightening analogy between ion transport
in nanochannels and transport in doped semi-conductors will be discussed.
