The basics of designing any programming interface apply: Model your domain well, choose good names, try to make things as self-explanatory as possible. Assumin

The basics of designing any programming interface apply: Model your domain well, choose good names, try to make things as self-explanatory as possible. Assuming you're tasked with doing a REST/JSON API, you should know both and be able to explain REST. Some things depend on the consumer of the API. For example if you're applying for a position to build an API for a Mobile App company, they'll place a bigger focus on reducing bandwidth, number of calls etc. A single-page-app company may be interested in long-polling/WebRTC/... A microservice company may place bigger emphasis on asynchronous processing with webhooks or so. Here is some advanced stuff that should "wow" someone (but, TBH, I wouldn't necessarily expect a junior dev to implement those): * Instead of REST, implement a GraphQL API * Document your API with Swagger/OpenAPI/RAML (+show some generated doc/client code) * Implement a CQRS API Good luck!