In this PhD thesis, the candidate has performed a metaphysical inquiry on the ontological structure of quantum field theory. This analysis s required since it is the rnost advanced physical theory concerned with the inherent structures of our reality. Unfortunately, this theory relies on the ill-defîned structures (both conceptual and mathematical) of quantum mechanics. After having convincingly refuted the widespread belief that an ontology of localizable particles is incompatible with the basic tenets of quantum field theory, the thesis compared the metaphysical pros and cons of two proposais: the so-called "Bell type quantum field theories", which associate an ontology of particles to (more or less) any regularized quantum field theory; in such theories the particles are non persistent in time (they can be created and annihilated) and evolve according to a non deterministic law , and the so called "Dirac sea models", that allow for ail ontology of persisting particles that move on continuous trajectories following a deterministic law. These models are based on a ontologically/technically consistent approach, thus they represent two viable alternative to quantum field theory.