  This is the author's PhD thesis. It is a contribution to categorical logic,
in particular to the theory of realizability toposes.
  While the tools of categorical logic have proven very successful in analyzing
and organizing proof theoretic realizability interpretations, it was remarked
by experts (notably Peter Johnstone) that the field of realizability toposes
itself was not clearly delineated, and lacked a powerful theory analogous to
that of Grothendieck toposes.
  The present work sets out to remedy this situation to a certain extent. We
argue that realizability toposes are best understood using Grothendieck
fibrations, and develop a framework of fibrational cocompletions, which allows
to view certain constructions from realizability in precise analogy to
constructions of presheaf and sheaf toposes. Using these techniques, and a
class of posetal fibrations that we call uniform preorders, we are able to give
an extensional characterization of partial combinatory algebras and of the
realizability toposes that are constructed from these algebras.
  Striving to develop the analogy to Grothendieck toposes further, we outline
how to apply our techniques on arbitrary base toposes, and give a decomposition
theorem for constant objects functors induced by triposes, analogous to the
known decompositions of geometric morphisms. Finally, we sketch an approach of
how to find a unified framework for Grothendieck toposes and realizability
toposes, based on the observation that uniform preorders can be identified with
preorders internal to a category of sheaves.
