  Although the laws of thermodynamics are well established for black hole
horizons, much less has been said in the literature to support the extension of
these laws to more general settings such as an asymptotic de Sitter horizon or
a Rindler horizon (the event horizon of an asymptotic uniformly accelerated
observer). In the present paper we review the results that have been previously
established and argue that the laws of black hole thermodynamics, as well as
their underlying statistical mechanical content, extend quite generally to what
we call here "causal horizons". The root of this generalization is the local
notion of horizon entropy density.
