I distinguish in this chapter between a weak and a strong form of ontological naturalism. Strong ontological naturalism is the view that all truths can be deduced, at least in principle, from truths about physical entities at the lowest level of organization, for example, truths about the elementary particles and forces. Weak ontological naturalism is the view that only physical properties can be causally efficacious. Strong ontological naturalism entails weak ontological naturalism, but not vice versa. I then argue that the (...) existence of a truth property is consistent with weak ontological naturalism, as truth is not causally efficacious. After considering several prominent theories of truth, I argue that the only theories of truth that are consistent with strong ontological naturalism are deflationary doctrines that deny that there is a substantial truth property, but these theories are not suitable adjoints to strong naturalism, as folks with strong naturalist inclinations normally want to posit the existence of substantial natural properties. Yet, if the truth property itself is insubstantial, so are claims about the existence of substantial natural properties. I conclude by arguing that the result that there cannot be a naturalistic theory of truth is unsurprising, as there are independent reasons for thinking that the basic semantic notions must be treated as irreducible, primitive properties. (shrink)