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This is a condensed overview. For the full specification, see hl7.org/fhir.

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What is FHIR?

FHIR (Fast Healthcare Interoperability Resources) is a standard for representing healthcare data as resources and exchanging them over HTTP via a REST API.

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Resources

A resource is a structured document with a known shape. FHIR defines several formats (JSON, XML, RDF). Zunder supports JSON and XML; this documentation uses JSON for examples. It might represent a person (Patient), an event (Encounter), or a clinical fact (Observation). Every resource has a resourceType, and most include an id and meta: 
{
  "resourceType": "Patient",
  "id": "example",
  "meta": {
    "versionId": "1",
    "lastUpdated": "2025-01-15T10:30:00Z"
  },
  "name": [{ "family": "Doe", "given": ["Jane"] }],
  "birthDate": "1990-03-15"
}
Three “infrastructure” resources appear constantly:
  • Bundle — groups multiple resources (search results, batches, transactions)
  • OperationOutcome — error and validation messages
  • CapabilityStatement — a server’s self-description (GET /metadata)

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REST API

FHIR uses standard HTTP methods:
OperationMethodURL pattern
CreatePOST/fhir/{type}
ReadGET/fhir/{type}/{id}
UpdatePUT/fhir/{type}/{id}
PatchPATCH/fhir/{type}/{id}
DeleteDELETE/fhir/{type}/{id}
SearchGET/fhir/{type}?param=value
Search returns a Bundle of type searchset. Parameters are typed — the same parameter name behaves differently depending on its type:
TypeExample
stringname=Doe (prefix match)
tokencode=http://loinc.org|29463-7
datebirthdate=ge2000-01-01
referencesubject=Patient/123
quantityvalue-quantity=gt5.0||mg
Common result parameters: _count, _sort, _include, _revinclude, _summary, _elements.

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Terminology

FHIR uses coded values extensively:
  • CodeSystem — defines a set of codes (LOINC, SNOMED CT, ICD-10, …)
  • ValueSet — a curated subset of codes allowed in a given context
  • Coding — one code from one system (system + code)
  • CodeableConcept — one or more Codings plus optional human-readable text
Servers expose terminology operations like $expand, $lookup, and $validate-code.

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Profiles and Extensions

Profiles (StructureDefinitions) constrain base resources for specific use cases — restricting cardinality, fixing values, or adding required elements. Extensions let you add data that isn’t in the base model, identified by a canonical URL.

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Key Concepts at a Glance

TermMeaning
BundleContainer for multiple resources
CompartmentScoped view of resources related to an instance (e.g. Patient/123/Observation)
ETag / If-MatchVersion-aware concurrency control
FHIRPathExpression language for navigating resources
Operation$-prefixed procedure beyond CRUD (e.g. $expand, $validate)
TransactionAtomic batch — all entries succeed or all fail

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Further Reading