The CDS Hooks API is a new specification that builds on FHIR, describing how EHRs can automatically invoke external platforms or apps. For the first time, by CDS Hooks invoking FHIR apps, EHR-only users can have insights relevant to the clinical situation and within their EHR workflow based on data sourced outside of it. CDS Hooks is potentially a powerful way to ensure that all patient data can be accessed via the EHR.