This study analyzes the legal requirements concerning discharges from ships, a matter that is characterized by a considerable degree of complexity. This complexity results, inter alia, from the highly technical nature of the applicable norms, but also from the fact that the relevant rules and principles are prescribed in a wide and often overlapping variety of instruments on different levels of law, namely public international law, European Union law (where applicable) and domestic law. Taking into account that the individual legal instruments within these sub-systems of law significantly differ in their spatial and substantive scopes and regulatory approaches, a risk of conflicts of norms exists both from a vertical (i.e., between different levels of law) and horizontal (i.e., between different instruments on the same level of law) perspective. This situation gives rise to legal uncertainties, which may ultimately threaten the lawful and effective application and implementation of the relevant norms. This study attempts to clarify the existing uncertainties and to suggest harmonized interpretations and applications of the pertinent rules and principles. It does not address the issue of pollution from ships in general, but focuses on three specific categories of vessel discharges, namely scrubber washwater, sewage and ballast water.