StructureDefinitions

StructureDefinition is a formal artifact in the HL7 FHIR specification that defines the structure, constraints, and semantics of data resources, data types, profiles, and extensions. It documents how a base resource or primitive type is extended or constrained to meet interoperability goals. StructureDefinition resources are used to publish profiles, which add constraints to existing resources, and extensions, which introduce new elements.

A StructureDefinition identifies its target through baseDefinition and declares its kind, such as resource, complex-type, primitive-type, or logical. It uses ElementDefinition components to describe individual elements by path, along with constraints such as min and max cardinalities, data types, and bindings to terminology. The resource includes a snapshot, a complete view of all elements, and a differential, which records only the constraints relative to the base.

Key features include element slicing, constraints expressed with FHIRPath expressions, and terminology bindings to ValueSets. Profiles can specify cardinality, data types, and required or optional bindings; extensions add elements to resources within defined boundaries. StructureDefinitions are designed to be machine-readable and are used by validators and tooling to check conformance.

Implementation and governance: StructureDefinitions are published by organizations and collaborators in registries and are adopted by the FHIR community to promote interoperability. They underpin standard profiles (such as US Core) and enable customized implementations while maintaining shared interoperability guarantees.

See also:

- FHIR

- StructureDefinition resource

- Profile (FHIR)

- Extension (FHIR)

- ElementDefinition

- ValueSet