  This thesis introduces the "method of structural refinement", which serves as
a means of transforming the relational semantics of a modal and/or constructive
logic into an 'economical' proof system by connecting two proof-theoretic
paradigms: labelled and nested sequent calculi. The formalism of labelled
sequents has been successful in that cut-free calculi in possession of
desirable proof-theoretic properties can be automatically generated for large
classes of logics. Despite these qualities, labelled systems make use of a
complicated syntax that explicitly incorporates the semantics of the associated
logic, and such systems typically violate the subformula property to a high
degree. By contrast, nested sequent calculi employ a simpler syntax and adhere
to a strict reading of the subformula property, making such systems useful in
the design of automated reasoning algorithms. However, the downside of the
nested sequent paradigm is that a general theory concerning the automated
construction of such calculi (as in the labelled setting) is essentially
absent, meaning that the construction of nested systems and the confirmation of
their properties is usually done on a case-by-case basis. The refinement method
connects both paradigms in a fruitful way, by transforming labelled systems
into nested (or, refined labelled) systems with the properties of the former
preserved throughout the transformation process.
  To demonstrate the method of refinement and some of its applications, we
consider grammar logics, first-order intuitionistic logics, and deontic STIT
logics. The introduced refined labelled calculi will be used to provide the
first proof-search algorithms for deontic STIT logics. Furthermore, we employ
our refined labelled calculi for grammar logics to show that every logic in the
class possesses the effective Lyndon interpolation property.
