IEC TR 62541-2:2016(E) is a Technical Report and describes the OPC unified architecture (OPC UA) security model. It describes the security threats of the physical, hardware, and software environments in which OPC UA is expected to run. It describes how OPC UA relies upon other standards for security. It provides definition of common security terms that are used in this and other parts of the OPC UA specification. It gives an overview of the security features that are specified in other parts of the OPC UA specification. It references services, mappings, and Profiles that are specified normatively in other parts of this multi-part specification. It provides suggestions or best practice guidelines on implementing security. Any seeming ambiguity between this part of IEC 62541 and one of the normative parts of IEC 62541 does not remove or reduce the requirement specified in the normative part. This second edition cancels and replaces the first edition of IEC TR 62541-2, published in 2010. This second edition includes no technical changes with respect to the first edition but a number of clarifications and additional text for completeness.