In this chapter, we have revisited the Telemetry service in OpenStack by scratching the surface of each sub-project including Ceilometer, Aodh, and Gnocchi. Although we did not cover all the available metrics within the latest releases of OpenStack, you should give more importance to understanding how the Telemetry module is being extended and works under the hood. We next expanded our monitoring vision by enforcing what was set up in the first part of the chapter with a sophisticated external monitoring solution: Nagios. You should be able to exploit its extreme benefits in terms of plugins and configuration flexibility. That opens the curtains to deliver a rich monitoring data where operators and administrators get more insight on the deployed OpenStack infrastructure. By the end of this chapter, we have looked at a few commons ways on how to take immediate actions and where to look once issues are raised...