  An outline of Morita's equilibrium ensemble approach to disordered systems is
given, and hitherto unnoticed relations to other, more conventional approaches
in the theory of disordered systems are pointed out. It is demonstrated to
constitute a generalization of the idea of grand ensembles and to be intimately
related also to conventional low--concentration expansions as well as to
perturbation expansions about ordered reference systems. Moreover, we draw
attention to the variational content of the equilibrium ensemble formulation. A
number of exact results are presented, among them general solutions for site--
and bond-- diluted systems in one dimension, both for uncorrelated, and for
correlated disorder.
