FHIR (Fast Healthcare Interoperability Resources), pronounced 'fire,' is a standard developed by HL7 International for exchanging healthcare information electronically using modern web technologies, REST APIs, JSON, XML, and OAuth2. FHIR R4 (Release 4) is the current normative version and is now mandated by US federal regulations for patient data access.\n\nFHIR defines over 140 data resources, structured data objects representing clinical and administrative concepts. Key resources include Patient, Encounter, Observation (lab results, vitals), Condition (diagnoses), MedicationRequest (prescriptions), Procedure, DiagnosticReport, DocumentReference, Practitioner, and Organization. Each resource has a defined structure, terminology bindings, and REST interactions (read, search, create, update, delete).\n\nFHIR APIs are accessed via standard HTTPS requests. A FHIR server exposes a base URL (the FHIR endpoint), and clients query it using FHIR search parameters. For example, to retrieve all observations for a patient, a client sends: GET [base]/Observation?patient=123.\n\nSMART on FHIR adds a security layer, standardizing how apps authenticate and request access to FHIR APIs using OAuth2 and OIDC. Epic, Cerner, and most major EHR vendors expose SMART on FHIR endpoints that allow third-party applications to access patient data with appropriate consent.\n\nThe CMS Interoperability and Patient Access Rule (CMS-9115-F) requires payers and health systems to expose FHIR R4 APIs for patient data access. The ONC 21st Century Cures Act Final Rule requires EHR vendors to support standardized FHIR APIs, making FHIR a regulatory requirement for health IT.