We describe the online event monitoring systems using ROOT [1] for the CDF
and DO collaborations in the upcoming Fermilab Tevatron runII. The CDF and
DO experiments consist of many detector subsystems and will run in a high r
ate large bandwidth data transfer environment. In the experiments, it is cr
ucial to monitor the performance of each subsystem and the integrity of the
data, in real time with minimal interruption. ROOT is used as the main ana
lysis tool for the monitoring systems and its GUI is used to browse the res
ults via socket, allowing multiple GUI client connections.