Many system administrators are responsible for monitoring numerous servers that have different critical roles and often run applications 24/7. Properly planning and selecting monitoring software is important to determine what and how to monitor servers effectively based on requirements, while considering scalability, cost, and support. Nagios is an open-source option that is well-documented, customizable through plugins, and easy to install, but configuration requires using templates and hierarchies to manage alerts and check intervals for large server pools.