This article focuses on the interpretation and application of law in WTO dispute settlement from the angle of legal certainty and predictability. An analysis of the interpretation of WTO law shows that in general it does not differ from the interpretation of other public international law as interpretative rules well known in international law are applied. This together with the consistence provided by the respect of earlier panel and Appellate Body reports safeguard legal certainty. Furthermore, legal certainty and predictability requires clarity in the law applicable in WTO dispute settlement, in particular as regards non-WTO law. It will be shown that apart from peremptory norms of public international law (ius cogens), the relevance of international law outside WTO law is limited. Non-WTO treaty law must not be applied except if referred to by WTO law or incorporated therein. Apart from that international law of any kind can only be considered when interpreting WTO law. In certain circumstances this applies even to non-WTO treaty law to which not all WTO members are parties. Due to the as yet limited importance of non-WTO law, legal certainty and predictability also depend on the issue of conflict of norms, which also is relevant as far as the interrelationship of the different WTO agreements is concerned. In this regard predictability and legal certainty cannot be fully reached.