  Model theoretic results such as Characterization and Definability give
important information about different logics. It is well known that the proofs
of those results for several modal logics have, somehow, the same 'taste'. A
general proof for most modal logics below first order is still too ambitious.
In this thesis we plan to isolate sufficient conditions for the
characterization and definability theorems to hold in a wide range of logics.
Along with these conditions we will prove that, whichever logic that meets
them, satisfies both theorems. Therefore, one could give an unifying proof for
logics with already known results. Moreover, one will be able to prove
characterization and definability results for logics that have not yet been
investigated. In both cases, it is only needed to check that a logic meets the
requirements to automatically derive the desired results.
