Check_MK is a complex addon for Nagios/Icinga and consists of three subprojects. The check and inventory system Check_MK is a general purpose Nagios plugin for retrieving data. It adopts a new approach for collecting data from operating systems and network components. It obsoletes NRPE, check_by_ssh, NSClient, and check_snmp. It has many benefits, the most important of which are significant reduction of CPU usage on the Nagios host and automatic inventory of items to be checked on hosts. The larger your Nagios installation is, the more helpful these improvements. The Nagios module "MK Livestatus" gives immediate and fast access to live status data of Nagios and to historic data in the Nagios logfile. It's a supported backend for many addons like NagVis, NagiosBP, Thruk, and others. The Nagios GUI "Check_MK Multisite" is a feature complete replacement for the classical Nagios GUI, and uses MK Livestatus as a backend. Multisite is very fast, and supports efficient distributed monitoring using Livestatus as transport layer.Changes: This release fixes some bugs. It also introduces trend analysis on filesystems.