This chapter distinguishes between the two how questions of normativity. The first (epistemological perspective) is concerned with finding out how to ascertain a legal norm. It relies on Kelsen’s understanding of ‘validity’ as a specific norm’s ‘existence’, which is determinable due to the doctrine of formal sources of law. The chapter discusses the following problems: how to differentiate a valid international legal norm from a mere act of will; how to ascertain a valid customary legal rule, a jus cogens rule, an erga omnes obligation, and how to ascertain rule’s invalidity based on its inefficacy. The second question (perspective of practical rationality) tries to elucidate how legal norms provide us with reasons for action. It is traditionally held that legal norms are ‘non-optional’ – that they preempt and exclude all the conflicting reasons that norm-subjects might have for the contrary behavior. This view is challenged and it is argued there is nothing special about the normativity of law, insofar as it competes with the normative force of other normative orders. This view opens the room for gradation of legal normativity, which is of particular importance for the discussion about relative normativity of international law and soft law.