IEC 62541-4:2015 defines the OPC Unified Architecture (OPC UA) Services. The Services described are the collection of abstract Remote Procedure Calls (RPC) that are implemented by OPC UA Servers and called by OPC UA Clients. This second edition cancels and replaces the first edition published in 2011. It constitutes a technical revision. It includes the following changes: - Update for 6.4 Redundancy. - Clarifications for Publish and Reconnect scenarios. - Handling of MonitoredItem changes in short network interruption scenarios. - Update for 6.1.3 Determining if a Certificate is Trusted. - Revised definition of parameters semaphoreFile and isOnline in Service RegisterServer. - Services ModifySubscription and ModifyMonitoredItems.