Many electronics products are used in humid environments. High humidity in combination with various contaminants will affect the surface insulation resistance on printed board assemblies and may cause current leakage or in worse case short circuit due to electrochemical migration. This chapter discusses the failure mechanism for electrochemical migration and how it is affected by common contaminants on assemblies. Test methods for measuring surface insulation resistance and for assessing the risk for electrochemical migration are reviewed, and their relevance is discussed.