This course clarifies the fundamental concepts behind the notion of Web services and presents 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 are discussed. Established, as well as recently proposed, standards and techniques (e.g., XML, SOAP, WSDL, REST, and some WS-* specifications), are examined in the context of this discussion in order to emphasize their scope, benefits, and shortcomings.