  Multiple orthogonal polynomials are traditionally studied because of their
connections to number theory and approximation theory. In recent years they
were found to be connected to certain models in random matrix theory. In this
paper we introduce the notion of a multiple orthogonal polynomial ensemble (MOP
ensemble) and derive some of their basic properties. It is shown that Angelesco
and Nikishin systems give rise to MOP ensembles and that the equilibrium
problems that are associated with these systems have a natural interpretation
in the context of MOP ensembles.
