API Standards Web service APIs are interfaces for data transmission or exchange using the Web’s HyperText Transfer Protocol (HTTP).4 An API client can obtain data from an API or post data to an API. Analysis and transformation software can obtain source data and contextual metadata from APIs, can post integrated data to APIs, or can serve APIs from which integrated data is obtained. There were 22,000 APIs registered with the ProgrammableWeb™ API Directory5 in June 2019, with the number growing at 220 APIs per month. Many domain-specific data structure standards (including FHIR, OFX, and the Open Banking Standard) define data structures used in APIs.