What is imagination? What makes it different to other kinds of mental states? Why might we think that, at least in some cases, imagining can give us knowledge? If this is right, then what can imagination teach us? Do dreams count as imaginative states? If they do, can dreams also give us knowledge? These are important questions in philosophy and psychology, and the philosophical literature is not conclusive on any of them. In this thesis, I offer some answers, with the aim of shedding light on how (and when) imagination can be a source of knowledge. This is a Doctoral Thesis in Philosophy at Stockholm University, Sweden 2021