REST, gRPC, SignalR and GraphQL. As .NET developers, we hear a lot about those four technologies to build API. However, it is not always crystal clear how to decide the one to pick for a use case. REST is a software architectural style. gRPC is a cross-platform open source Remote Procedure Call (RPC) framework. SignalR is a free and open-source software library. GraphQL is an open-source data query and manipulation language for APIs. In this talk, we discuss their core concepts and the problems they are trying to solve. Finally, we propose a decision-tree to help you decide the right one for your use case.