Nitrification is widespread in drinking water distribution systems that use chloramination as a residual disinfectant. Factors affecting the likelihood of nitrification episodes include pH, monochloramine, trihalomethane (THM), and ammonia concentrations. This research advanced the understanding of the role THMs play in inhibiting distribution system nitrification, placed THMs within the context of the other key factors that determine the risk of nitrification, and developed a fundamental framework for assessing nitrification risks in a way that provides practical guidance to utilities. Laboratory‐scale annular reactor experiments demonstrated the potential of THMs to inhibit nitrification. The nitrification index (NI) was developed from the fundamental kinetics of nitrifier growth and inactivation by monochloramine and THMs. A linear relationship was demonstrated between the rate of nitrification onset and NI values. Tracking the NI over time in distribution systems may provide an effective early warning system that would permit timely adjustments before nitrification begins.