This chapter covers types of corrosion threat, information required to carry out corrosivity assessments, hotspot identification and system corrosivity evaluation for both routine and non-routine operations. It includes considerations on competency levels required for carrying out corrosivity assessment. Representative water chemistry should be made available. This is critical for an effective internal corrosion threat assessment. The gas analysis should include the percentage of the acid gases (Carbon dioxide and Hydrogen Sulfide) and Oxygen. The most common non-routine operations are typically well interventions which may lead to backflow of well treatment fluids into the pipeline. The marine splash zone is the area that is alternately immersed due to the action of waves and tides. A landfall is a critical part of a submarine pipeline system going onshore which requires a thorough evaluation. From an engineering point of view, the knowledge of corrosion severity presents a convenient means of describing or classifying corrosive conditions as a basis for managing corrosion in pipelines.