Model Driven Development (MDD) has emerged as a new road to software development industrialization. The most well-known realization of MDD is the Model Driven Architecture (MDA). The essence of MDA is the metamodel MOF (Meta Object Facility) allowing interoperability of different kind of artifacts from multiple technologies. It is important to formalize and reason about MOF metamodels properly. In this paper, we propose a rigorous framework for reasoning about “correctness” of metamodels. Our main contribution is the integration of MOF metalanguage with formal specification languages based on the algebraic formalism. We define NEREUS, a formal metamodeling language, and processes for reasoning about MOF-like metamodels such as ECORE metamodels. The paper describes a set of tools developed to make formal metamodeling feasible in practice.