This article aims to contribute to the largely neglected issue of whether metaphysical grounding – the relation of one fact’s obtaining in virtue of the obtaining of some other (or others) – can be given a reductive account. I introduce the notion of metaphysically opaque grounding, a form of grounding which constitutes a less metaphysically intimate connection than in standard cases. I then argue that certain important and interesting views in metaphysics are committed to there being cases of opaque grounding (...) and demonstrate that four representative accounts of grounding available in the literature are unable to accommodate such cases. This is argued to constitute a problem for those accounts that is likely to extend to other possible reductive accounts of grounding that employ the popular strategy of explaining grounding in terms of other hyperintensional phenomena. Unless the reductionist is willing to opt for some sophisticated modalist account, the possibility of opaque grounding cases thus provides indirect support for primitivism about grounding, a view that has previously been widely embraced but rarely supported by argument. (shrink)