Very often, we are unsure about what we ought to do. Under what conditions should we help to improve the lives of distant strangers rather than those of our family members? At what point does an embryo or foetus become a person, with all the rights that that entails? Is it ever permissible to raise and kill non-human animals in order to use their meat for food? Sometimes, this uncertainty arises out of empirical uncertainty: we might not know to what extent non-human animals feel pain, or how much we are really able to improve the lives of distant strangers compared to our family members. But this uncertainty can also arise out of fundamental normative uncertainty: out of not knowing, for example, what moral weight the wellbeing of distant strangers has compared to the wellbeing of our family; or whether non-human animals are worthy of moral concern even given knowledge of all the facts about their biology and psychology. In fact, for even moderately reflective agents, decision-making under normative uncertainty is ubiquitous. Given this, one might have expected philosophers to have devoted considerable research time to the question of how one ought to take one’s normative uncertainty into account in one’s decisions. But the issue has been largely neglected. This thesis attempts to begin to fill this gap. It addresses the question: what ought one to do when one is uncertain about what one ought to do? It develops a view that I call metanormativism: the view that there are second-order norms that govern action that are relative to a decision-maker’s uncertainty about first-order normative claims. In consists of two distinct parts. The first part (Chapters 1-4) develops a general metanormative theory. I argue in favour of the view that decision-makers should maximise expected choice-worthiness, treating normative uncertainty analogously with how they treat empirical uncertainty. I defend this view at length in response to two key problems, which I call the problems of merely ordinal theories and the problem of intertheoretic comparisons. The second part (Chapters 5-7) explores the implications of metanormativism for other philosophical issues. I suggest that it has important implications for the theory of rational action in the face of incomparable values, for the causal/evidential debate in decision-theory, and for the value we should ascribe to research into moral philosophy.