Abstract
Schema.org is an initiative by the major search engine providers Bing, Google, Yahoo!, and Yandex that provides a collection of ontologies which webmasters can use to mark up their pages. Schema. org comes without a formal language definition and without a clear semantics. We formalize the language of Schema. org as a Description Logic (DL) and study the complexity of querying data using (unions of) conjunctive queries in the presence of ontologies formulated in this DL (from several perspectives). While querying is intractable in general, we identify various cases in which it is tractable and where queries are even rewritable into FO queries or datalog programs.