We describe the online monitoring system of the CDF II experiment
designed to check the data quality of all subsystems in real-time.  A subset
of the events accepted by the highest trigger level is made available to 10
analysis programs which check the data quality on an event-by-event basis
and produce diagnostic histograms.  The monitoring results are distributed
to the user via a client-server scheme.  The display clients feature an
intuitive GUI which allows to browse and request the available results.  The
monitoring package is coded in C++ and makes use of the ROOT analysis
framework which offers histogramming methods, networking classes and
graphics handling.  We report about the design and the implementation of the
monitoring system and discuss our experience during the first months of
operation.