As the world witnesses an era of Software Defined Vehicle (SDV) and Internet of Things (IoT), the vehicles start becoming more intelligent, bringing in more software complexity than ever. This includes the Electrical/Electric (E/E) systems embedded inside today’s SDVs. The Electronic Control Unit (ECU) is now capable of running software which can not only communicate with other ECUs within the vehicle, but also to the outside world using protocols which are already in use within the Information Technology (IT) world for a long time. This lead to the emergence of an off-board vehicle Application Programming Interface (API) concept which allows external devices like smartphones or servers to access vehicle information such as those of sensors and other peripherals inside the vehicle. The Connected Vehicle Systems Alliance (COVESA) came up with an approach of defining a catalog for vehicle signals using the Vehicle Signal Specification (VSS) initiative. The World Wide Web Consortium (W3C) then brings this into use by defining Vehicle Information Service Specification (VISS). But the current VISS standard, namely version 2, defines transport bindings for WebSocket, Hypertext Transfer Protocol (HTTP) and Message Queuing Telemetry Transport (MQTT). These are well established protocols in the IT world and also suitable for off-board use cases. But if we want to use such an API concept inside the vehicle, so that the internal applications can also take advantage of a standardized catalog of signals and be reused between different vehicles, we need to consider protocols already in use inside vehicles as well. Scalable service-Oriented MiddlewarE over IP (SOME/IP) is one such example. Therefore, in this thesis, we elaborate the concept of a vehicle API, look at related technologies in more detail. Later, we take a look at one of the suitable candidates for an in-vehicle protocol, namely SOME/IP and see it’s respective trade-offs. Finally, we conceptualize and realize a Vehicle API concept for onboard usage.