Caustic injection is used to reduce refinery crude-column-overhead hydrochloric acid corrosion. Such injection systems are considered to be critical equipment because failures can potentially have catastrophic results. Accordingly, it is necessary to rigorously design caustic injection systems to assure correctness and reliable performance. A series of case studies is presented to demonstrate the advantages and disadvantages of different designs and materials that have been used. Failures encountered in caustic injection systems are also discussed. This paper draws on refinery experience to provide a comprehensive approach to design, operation, and troubleshooting.