Nagios Core is the monitoring and alerting engine that serves as the primary application around which hundreds of Nagios projects are built. It serves as the basic event scheduler, event processor, and alert manager for elements that are monitored. It features several APIs that are used to extend its capabilities to perform additional tasks, is implemented as a daemon written in C for performance reasons, and is designed to run natively on Linux/*nix systems.

Nagios XI is the most powerful IT infrastructure monitoring solution on the market. Nagios XI extends on proven, enterprise-class Open Source Nagios Core components to deliver the best monitoring solution for today's demanding organizational requirements. Designed for scalability and flexibility, XI is designed to make problematic IT monitoring tasks simple, while retaining the powerful attributes of its enterprise-class foundation blocks.