Narrow mental content, if there is such a thing, is content that is entirely determined by the goings-on inside the head of the thinker. A central topic in the philosophy of mind since the mid-1970s has been whether there is a kind of mental content that is narrow in this sense. It is widely conceded, thanks to famous thought experiments by Hilary Putnam and Tyler Burge, that there is a kind of mental content that is not narrow. But it is often maintained that there is also a kind of mental content that is narrow, and that such content can play various key explanatory roles relating, inter alia, to epistemology and the explanation of action. This book argues that this is a forlorn hope. It carefully distinguishes a variety of conceptions of narrow content and a variety of explanatory roles that might be assigned to narrow content. It then argues that, once we pay sufficient attention to the details, there is no promising theory of narrow content in the offing.