  The basic approximations leading to non-relativistic and relativistic nuclear
y and x scaling are recalled. The general and systematic features of y-scaling
structure functions are pointed out, and a recently proposed novel approach to
y-scaling, based on a global scaling variable, y_G, which incorporates the
effect of the momentum dependence of the nucleon removal energy, and therefore
allows the establishment of a direct link between scaling functions and
momentum distributions, is illustrated and applied to the analysis of a large
body of data. The phenomenon of nuclear x-scaling, based on a proper analysis
of nuclear quasi-elastic data in terms of the Bjorken scaling variable x_B, is
shown to occur at high values of the four-momentum transfer Q^2; the usefulness
of nuclear x scaling is pointed out.
