The initial conversion of ammonium to nitrite by Nitrosomonas has traditionally been regarded as the rate-limiting step for nitrification metabolism. This perspective implicitly assumes that subsequent oxidation of nitrite by Nitrobacter occurs more rapidly, and that NO2 concentrations are consequently maintained at low, sub-mg/L values. However, numerous bench- and full-scale nitrification systems have reportedly encountered elevated nitrite concentrations. Several concerns are generated by this circumstance, including: a) an increased chlorine demand, b) an increased effluent nitrogenous oxygen demand, c) potential nitrite toxicity, and d) possible nitrosamine formation. This paper consequently provides an overview of seven conditions which could lead to elevated nitrite occurrence in biological nitrification systems.