In this thesis we investigate automorphisms of partial combinatory algebras and construct realizability models of constructive set theory. After some introductory and background material in chapters 1 and 2, we define in chapter 3 a generalisation of Kripke and realizability models of intuitionistic logic that we call Kripke realizability models. In chapters 4, 6 and 7 we then develop various realizability models of constructive set theory. We show in chapter 5 how to use these techniques to investigate the automorphisms of some partial combinatory algebras. In chapter 8 we use a Kripke realizability model to show that a property known as the existence property does not hold for the set theory CZF.