Casing corrosion is a phenomenon which is mostly overlooked during well design and construction. Many operators in climates ranging from desert to arctic conditions have experienced aggressive corrosion of the well conductors and casings. This paper describes some field examples for which the root cause of the attack has been established. The corrosion process has been modelled to provide a basis for ranking the condition of other wells in the field and prediction of the course of future corrosion attack. Corrosion mitigation steps for new wells are proposed (lessons learnt) and potential remediation steps for existing wells are suggested.