Recognizing the potential of large databases within HMOs for the evaluation of vaccine safety, the Centers for Disease Control and Prevention (CDC) funded the Vaccine Safety Datalink project, linking outcome and vaccine exposure information at Group Health Cooperative of Puget Sound, Kaiser Permanente Northwest, Kaiser Permanente Medical Care Program Northern California, and Southern California Kaiser Permanente. Integral to the Vaccine Safety Datalink Project was the development of Immunization Tracking Systems at each site; this report describes the effort required to establish these tracking systems. Essential requirements are the methods used to insure data quality and to educate system users. Tracking systems can be a valuable means for assessing vaccine coverage, evaluating barriers to complete immunization, and studying the effectiveness of interventions design to improve immunization coverage. Finally, we report on recent efforts to link HMO Immunization Tracking Systems with developing regional tracking systems.