We address in this chapter the problem of the automated discovery and composition of Web Services. Now, Service-oriented computing is emerging as a new and promising paradigm. However, selection and composition of Services to achieve an expected goal remain purely manual and time consuming tasks. Basing our approach on domain concept definitions thanks to an Ontology, we develop here an algebraic approach that enables to express formal definitions of Web Service semantics as well as user information needs. Both are captured by the means of algebraic expressions of ontology properties. We present an algorithm that generates efficient orchestration plans, with characteristics of optimality regarding Quality of Service. The approach has been validated by a prototype and an evaluation in the case of an Health Information System