In this tutorial, we will show you how to install Nagios on Debian 9. Nagios is an open-source monitoring tool for servers, network devices, and applications. It helps you monitor the services, resources, and applications that run on your server. It can also be used to monitor Linux and Windows servers as well as web applications, Linux/Unix applications, and Windows applications. Some of the features include parallelized service checks, a plugin system, notifications via email, pager or user-defined service, a web interface for viewing status, log files, problem history with a notification system, and much more.