Nitrite-nitrogen seldom exceeds 1.0 milligram per liter (mg/L) in a properly controlled nitrification process. As soon as ammonia is oxidized to nitrite by ammonia oxidizing bacteria (AOB), the nitrite is oxidized to nitrate by nitrite oxidizing bacteria (NOB). Under certain environmental conditions, such as a poorly established or inhibited NOB population, the nitrite concentration will increase. High nitrite concentrations can interfere with disinfection and cause violations of effluent bacteria limits, result potentially in whole effluent toxicity (WET) failures, and cause effluent pH violations. An abnormal increase in secondary effluent nitrite is referred commonly as “nitrite lock”. It is discussed frequently in the Water Environment Federations’ Technical Discussion Forum and is not understood fully. This paper will explain nitrite lock, present case studies from four treatment plants, and outline operational strategies for preventing and controlling nitrite lock.