  We propose stratified institutions (a decade old generalised version of the
theory of institutions of Goguen and Burstall) as a fully abstract model
theoretic approach to modal logic. This allows for a uniform treatment of model
theoretic aspects across the great multiplicity of contemporary modal logic
systems. Moreover Kripke semantics (in all its manifold variations) is captured
in an implicit manner free from the sometimes bulky aspects of explicit Kripke
structures, also accommodating other forms of concrete semantics for modal
logic systems. The conceptual power of stratified institutions is illustrated
with the development of a modal ultraproducts method that is independent of the
concrete details of the actual modal logical systems. Consequently, a wide
array of compactness results in concrete modal logics may be derived easily.
