A subway service diversion takes place when New York City Transit (Transit) must close all or a part of a subway line for capital projects or maintenance. For the period January 1, 2009, through July 14, 2010, Transit records show 3,332 service diversions were underway in various phases. Service diversions, while necessary, can be an inconvenience to riders and can have an adverse economic impact on businesses. In a joint audit with New York City Comptroller John Liu, we found that Transit has a number of policies and procedures for managing and controlling subway diversions. However, we also found that more needs to be done. In particular, diversion costs were not adequately monitored, daily work on diversions often started late and ended early, and the public was not adequately informed about diversions. These conditions add substantially to project cost, further inconvenience riders, and cause economic hardship to affected businesses.