This European Standard covers the general principles of cathodic protection when applied in seawater, brackish waters and marine mud. It is intended to be an introduction, to provide a link between the theoretical aspects and the practical applications, and to constitute a support to the other European Standards devoted to cathodic protection of steel structures in seawater. It includes criteria for protection, reference electrodes, design considerations, and secondary effects of cathodic protection. This European Standard directly covers the applications of cathodic protection in seawater not addressed by specific standards. For reinforced concrete structures exposed to seawater, EN 12696 shall be applied.