This book presents a methodology to model and specify the data aspect of Web services, as it is overlooked by current standards for specifying Web services. The formal specification enables verification of service behavior, and the proposed methodology is based on formal methods and design-by-contract techniques. The Web has evolved from an information sharing medium to a wide-scale environment for sharing capabilities or services. Currently, URLs not only point to documents and images, but are also used to invoke services that potentially change the state of the Web. Major online organizations today, such as Amazon, PayPal and FedEx, provide services for users and consumers. They also allow third-party vendors to resell their services. In both cases, this requires precise and complete specification of service offerings. Several online discussions demonstrate the challenges faced by these organizations and others while describing their data-centric Web services. These challenges surrounding data specification can lead consumers to use a service erroneously. Case studies demonstrate how formal methods, and specifically design-by-contract techniques, can be leveraged to address the lack of formal specification of data when it comes to developing Web applications such as Amazon and PayPal.