The author presents a precise notion of a realization (or simulation) of one concurrent system by another, and studies the relations of modular concurrent systems and non-persistent (i.e. With conflicts) concurrent systems in an automata theoretical style. He introduces a conception of realization that obeys three requirements: it allows for proper hierarchies in certain classes of concurrent systems; it allows for normal-form theorems, and the standard constructions of the literature remain realizations in formal concept; and it clarifies some counterintuitive examples. Further, although this realization conception is developed to translate the computational aspects of concurrent systems, it also gives a formal tool for the handling of synchronization problems. 38 references.