The purpose of this NACE International technical report is to provide basic information regarding the mechanisms involved with cathodic protection (CP) shielding for external coatings applied to carbon steel pipelines, and the manner in which the breakdown of coating systems may or may not contribute to CP shielding. It is intended to be a basic reference from which the tendency of a coating system to fail and shield CP may be understood on a conceptual basis. Non-corrosion-related pipeline protection systems, including concrete weight coating, polyethylene encasement, insulation, and mechanical pipeline protection systems, are outside the scope of this report. End users of this report may include pipeline designers, corrosion engineers, integrity management specialists, regulators, CP service providers, coating suppliers, coating service providers, equipment suppliers, and pipeline/facility owners.