A standard account of understanding—one that is especially prevalent in the philosophy of science—treats understanding as essentially a type of knowledge—viz., knowledge of causes. Unfortunately, this proposal is untenable, in that there are cases of genuine understanding where the relevant knowledge is lacking, and cases where the relevant knowledge is present but understanding is lacking. In light of these problems, I propose an alternative view which treats understanding as a kind of cognitive achievement. In recent work, however, Stephen Grimm has argued for an adapted version of the view that understanding is constituted by knowledge of causes, one that appeals to a kind of knowledge of causes which is non-propositional. I argue that Grimm's proposal, while admittedly ingenious, does not stand up to close scrutiny.