As the use of ozone begins to be incorporated into water treatment facilities for Cryptosporidium inactivation, the issue of dealing with high ozone residuals at the end of the ozone contactor will need to be addressed. Historically, the ozone residual at the end of the contactor was below 0.05 mg/L for inactivation of Giardia. The higher CT values being used as guidelines for Cryptosporidium inactivation can drive this value above 0.20 mg/L. The higher residuals can result in ozone off-gassing as the water passes through downstream processes with subsequent ambient ozone levels above safe operating guidelines. Both chemical addition to destroy ozone residual and design issues to mitigate residual carry-over can be used to alleviate this problem. Both of these options have been explored through pilot and full-scale chemical trials and engineering retrofits of existing ozone contactors. Both issues are presented with discussions on theory and application. Includes 8 references, table, figures.