  The Kondo effect involves the formation of a spin singlet by a magnetic
impurity and conduction electrons. It is characterized by a low temperature
scale, the Kondo temperature, $T_K$, and an associated long length scale,
$\xi_K = \hbar v_F/(k_BT_K)$ where $v_F$ is the Fermi velocity. This Kondo
length is often estimated theoretically to be in the range of .1 to 1 microns
but such a long characteristic length scale has never been observed
experimentally. In this review, I will examine how $\xi_K$ appears as a
crossover scale when one probes either the dependence of physical quantities an
distance from the impurity or when the impurity is embedded in a finite size
structure and discuss possible experiments that might finally observe this
elusive length scale.
