The thesis investigates metaphysical models of spin as a physical property instantiated by microphysical systems as described in non-relativistic quantum mechanics. The thesis is dived into two parts. The first part concerns foundational issues in meta-metaphysics. In particular, the author defends a naturalized approach to metaphysics, according to which metaphysical investigations have to be motivated and supported by our current best scientific theories. Furthermore, it is argued that naturalized metaphysics is in tension with (standard presentations of) scientific realism, and possible ways out from the tension are outlined. In the second part, the author investigates how spin has to be metaphysically modelled. After having introduced how spin is characterised in non-relativistic quantum mechanics and which approach to the metaphysics of quantum mechanics is assumed, it is argued that scientific realists must commit to spin’s existence as a physical property. Then, the author introduces the recent debate on quantum indeterminacy, and he presents new arguments against a widely shared approach to such indeterminacy. In the last part of the thesis, the author introduces an original characterisation of spin as a property through the determinable-determinate machinery. Such metaphysical machinery is instrumental to present a taxonomy of possible metaphysical models. Finally, a new and original view about spin is introduced and it is defended as a genuine contender in the debate which deserves more attention from the literature.