With the development of the web, a huge number of services available on the web have been published. These web services operate in several application domains like concurrent engineering, semantic web, system engineering or electronic commerce. Moreover, due to the ease of use of the web, the idea of composing these web services to build composite ones defining complex workflows arose. Even if several industrial standards providing specification and/or design XML-oriented languages for web services compositions description, like BPEL, CDL, OWL-S, BPMN or XPDL have been proposed, the activity of composing web services remains a syntactically based approach. Due to the lack of formal semantics of these languages, ambiguous interpretations remain possible and the validation of the compositions is left to the testing and deployment phases. From the business point of view, customers do not trust these services nor rely on them. As a consequence, building correct, safe and trustable web services compositions becomes a major challenge.