This course will describe the fundamental concepts behind the notion of Web services and present them as the natural evolution of conventional middleware, necessary to meet the challenges of the Web and of B2B application integration. The main objectives of Web services, the challenges that must be faced to achieve them, and the opportunities provided by this technology will be discussed. Established, as well as recently proposed, standards and techniques (e.g., XML, SOAP, WSDL, BPEL, REST, various WS-* specifications), are going to be examined in the context of this discussion in order to emphasize their scope, benefits, and shortcomings.