Making use of available services when building Web applications is a major challenge for today's developers. I address this challenge by using a declarative interface for data-centric Web services (aka data services), which are published as queries over a source schema. Programmers simply write queries over the source schema and rely on the system to automatically translate them to calls to existing data services. Thus, programmers can focus on extracting the data they need, without having to understand the definition or the implementation of each individual service.

This dissertation discusses the main underlying technical problem, that of deciding whether a query can be translated into service calls. We consider two settings: when the system cannot do any post-processing and hence can issue only one service call (I call that expressibility) and when it is able to issue several calls and combine the results (I call it support). Expressibility and support are studied both for services that are listed individually and for compactly represented services (using grammar-like or Datalog formalisms).

I also present contributions to extending the underlying service infrastructure with new features, several of which were added to the Distributed XQuery (DXQ) framework. DXQ is an XML query and scripting language with support for side effects, distribution, parallelism, which I also used as implementation platform for work flow languages.