The specification of complex systems is usually done by the 'divide and conquer' idea: the system is divided into smaller, less complex components that are developed separately and then merged in some way to form the specification of the whole system. The main aim of this paper is to provide an approach to the parallel composition of graph grammars, formalizing the intuitive idea of ' divide and conquer' described above, This parallel composition of graph grammars provides a suitable formalism for the specification of concurrent systems based on the specifications of their components. 'Dividing' is formalized by special graph grammar morphisms, called specialization morphisms. These morphisms also describe structural and behavioural compatibilities between graph grammars. As a main result, we characterize the parallel composition as the pullback in the category of graph grammars.
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