Motivated in part by the difficulties facing a quantum theory of gravity, Nobel Prize laureate G.'t Hooft has argued that quantum mechanics cannot provide a definitive description of the microscopic world. 't Hooft argues that, underlying quantum mechanics, a deeper level theory must exist, which must be deterministic, as opposed to the fundamental indetermism of quantum theory. Moreover, standard quantum mechanics must emerge as an effective theory out of this more fundamental theory. We will first review 't Hooft's arguments. Then we will briefly present our own contribution to this research line.