The design engineer and owner have multiple issues to address when planning to use an ozone process in a water or wastewater treatment plant. Considerations include feed-gas properties and equipment, ozone generator features and size, ozone contactor type and configuration, and methods of off-gas ozone destruction. Many successful ozone systems are in use today.Where performance has suffered, deficient feed-gas treatment often is the problem. The feed-gas to the ozone generator must be oil-free, particulate-free, and dry. In this paper, the factors involved in selecting and operating feed-gas equipment are presented.