HL7 is a health care application protocol accredited as a Standard by the American National Standards Institute (ANSI). This Standard covers implementation of communications between immunization service providers and immunization registries, using the HL7 Version 2.3.1 protocol. The Standard covers the events that trigger communication, the batching of transactions, together with the structure and content of electronic messages. This includes the data segments and data elements that are mandatory (required), optional or conditional (required, based on a condition), and relevant usage notes in the Australian health environment. The Standard provides consistent use of data definitions as well as commentary and references to the International Organization for Standardization (ISO), National Health Data Dictionary (NHDD).