HL7 (Health Level Seven International) is the standards body that produces the most widely used healthcare data exchange standards in the world. HL7 v2, first published in 1987 and continuously updated through v2.9, is deployed in virtually every hospital and clinic on the planet. It defines structured message formats for exchanging patient demographics, lab orders, lab results, radiology orders, clinical documents, financial data, and scheduling information between EHRs, lab systems, imaging systems, pharmacy systems, and ancillary applications.\n\nHL7 v2 messages are pipe-delimited text files that travel over MLLP (Minimum Lower Layer Protocol) on TCP/IP networks, or increasingly over HTTPS. Each message begins with an MSH (Message Header) segment that identifies the sending application, receiving application, message type, and timestamp. Common message types include ADT (Admit/Discharge/Transfer), ORM (Orders), ORU (Observation Results), MDM (Medical Document Management), DFT (Detailed Financial Transactions), SIU (Scheduling), and MFN (Master Files).\n\nHL7 v3 and CDA (Clinical Document Architecture) are XML-based standards used for clinical summaries, referral letters, and public health reporting. CDA documents are used in transitions of care, meaningful use requirements, and clinical quality reporting.\n\nHL7 interfaces are built and maintained using integration engines, middleware platforms that route, transform, and monitor HL7 messages between systems. The most common integration engines include InterSystems IRIS and HealthShare, Mirth Connect, Cloverleaf, and Rhapsody.