Mathematical logic and computer science have driven the design of a growing number of logics and related formalisms such as set theories and type theories. In response to this population explosion, logical frameworks have been developed as formal meta-languages in which to represent, structure, relate and reason about logics. Research on logical frameworks has diverged into separate communities, often with conflicting backgrounds and philosophies. In particular, two of the most important logical frameworks are the framework of institutions, from the area of model theory based on category theory, and the Edinburgh Logical Framework LF, from the area of proof theory based on dependent type theory. Even though their ultimate motivations overlap -for example in applications to software verification -they have fundamentally different perspectives on logic. In the current paper, we design a logical framework that integrates the frameworks of institutions and LF in a way that combines their complementary advantages while retaining the elegance of each of them. In particular, our framework takes a balanced approach between model theory and proof theory, and permits the representation of logics in a way that comprises all major ingredients of a logic: syntax, models, satisfaction, judgments and proofs. This provides a theoretical basis for the systematic study of logics in a comprehensive logical framework. Our framework has been applied to obtain a large library of structured and machine-verified encodings of logics and logic translations. † This work was developed over the course of five years, during F. Rabe 948 A similar difference in primacy is found when looking at families of logics. For example, the model-theoretical view takes precedence in description logic (see, for example, Brachman and Schmolze (1985) and Baader et al. (2003)), where the model theory is fixed and proof theory is studied chiefly to provide reasoning tools. This view is arguably paramount as it dominates accounts of (classical) first-order logic and most uses of logic in mathematics. On the other hand, the proof-theoretical view takes precedence in, for example, higher-order logic (see Church (1940) for proof theory and Henkin (1950) for model theory); and some logics such as intuitionistic logics (Brouwer 1907) are explicitly defined by their proof theory.At the level of logical frameworks, proof-theoretical principles have been integrated into model-theoretical frameworks in several ways. Parchments are used in Goguen and Burstall (1986) and Mossakowski et al. (1997) to express the syntax of a logic as a formal language. And, for example, in Meseguer (1989), Fiadeiro andSernadas (1988) and Mossakowski et al. (2005), proof theory is formalised in the abstract style of category theory. These approaches are very elegant, but often fail to exploit a major advantage of proof theory -the constructive reasoning over concrete syntax.Model-theoretical principles have also been integrated into proof-theoretical frameworks, albeit to a lesser extent. Fo...