  Jacob Bekenstein's identification of black hole event horizon area with
entropy proved to be a landmark in theoretical physics. In this paper we trace
the subsequent development of the resulting generalized second law of
thermodynamics (GSL), especially its extension to incorporate cosmological
event horizons. In spite of the fact that cosmological horizons do not
generally have well-defined thermal properties, we find that the GSL is
satisfied for a wide range of models. We explore in particular the case of an
asymptotically de Sitter universe filled with a gas of small black holes as a
means of casting light on the relative entropic 'worth' of black hole versus
cosmological horizon area. We present some numerical solutions of the
generalized total entropy as a function of time for certain cosmological
models, in all cases confirming the validity of the GSL.
