Answer - There are three modes of communication in WCF: One-Way In the One-Way operation mode, the client will send a request to the server and does not care whether it is success or failure of service execution. There is no return from the server side, it is one-way communication. Request-Reply When as client makes a request to the WCF service and client will wait to get a response from service (till receiveTimeout). After getting the response it will start executing the rest of the statement. Callback It is the service to call the client. In which, service will act as client and client will act as service.