Chemical reactions can be modeled and explored through systems of rule-based rewriting of molecular graphs. These Graph Transformation systems require (1) rules that prescribe how a pattern in the input graph is to be changed within a larger substrate graph and (2) a definition of a pattern match. In DPO graph rewriting, both facets are expressed in terms of morphisms, i.e., maps, between graphs, providing a sound mathematical foundation in category theory. A rule is specified as a so-called span, p = (L Therefore, it also unambiguously determines the ITS graph Υ(G, H, α) of the reaction.