The idea of determinatio – first identified and analysed in natural law theory – is crucial for understanding international human rights adjudication. Human rights, as they appear formulated in international human rights treatises and declarations, require specification, implementation, concretisation, i.e., determinatio, at the domestic level. I argue that there are good reasons for this to be so. One such reason is that determinatio allows for the application of a norm to be sensitive to the particular circumstances in which it takes place. Determinatio entails deference in human rights adjudication, the latter being the legal consequence of the reasonable space for discretion granted to states which is entailed by determiatio in international human rights law. Close attention to determinatio allows us to see well-known doctrines of deference (such as the doctrine of the margin of appreciation, of regional consensus and of incrementalism) in a different light—not as concessions to state sovereignty, but as grounded on reasons internal to the legal practice of human rights law, of which determinatio is an integral part.