  A necessary condition for the validity of the holographic principle is the
holographic bound: the entropy of a system is bounded from above by a quarter
of the area of a circumscribing surface measured in Planck areas. This bound
cannot be derived at present from consensus fundamental theory. We show with
suitable {\it gedanken} experiments that the holographic bound follows from the
generalized second law of thermodynamics for both generic weakly gravitating
isolated systems and for isolated, quiescent and nonrotating strongly
gravitating configurations well above Planck mass. These results justify
Susskind's early claim that the holographic bound can be gotten from the second
law.
