The Continua Design Guidelines (CDG) defines a framework of underlying standards and criteria that ensure the interoperability of devices and data used for personal connected health services. It also contains design guidelines (DGs) that further clarifies the underlying standards or specifications by reducing options or by adding missing features to improve interoperability. This specification focuses on the personal health devices interface (PHD-IF). Recommendation ITU-T H.811 is part of the "ITU-T H.810 interoperability design guidelines for personal connected health systems" subseries that covers the following areas: - ITU-T H.810 - Interoperability design guidelines for personal connected health systems: System overview - ITU-T H.811 - Interoperability design guidelines for personal connected health systems: Personal health devices interface design guidelines - ITU-T H.812 - Interoperability design guidelines for personal connected health systems: Services interface design guidelines - ITU-T H.812.1 - Interoperability design guidelines for personal connected health systems: Services interface: Observation upload capability - ITU-T H.812.2 - Interoperability design guidelines for personal connected health systems: Services interface: Questionnaires capability - ITU-T H.812.3 - Interoperability design guidelines for personal connected health systems: Services interface: Capability exchange capability - ITU-T H.812.4 - Interoperability design guidelines for personal connected health systems: Services interface: Authenticated persistent session capability - ITU-T H.813 - Interoperability design guidelines for personal connected health systems: Healthcare information system interface design guidelines