Uncertainty, incompleteness, and imprecision are common characteristics of the data and knowledge that users deal with in a wide range of domains and applications. Exploiting these uncertain data sources and services to their full potential raises important research challenges that relate to the different phases of their lifecycle. Modeling and representing the semantics of Web data sources and services is the first step toward the automation of their different related activities such as data and service querying, selection, integration, ranking, and composition. As the uncertainly of data and services could impact these activities, it should be considered as an integral part of data and service descriptions. The existing Web modeling languages and standards should be extended and used to represent the uncertainty of data and services.