In this paper, we present the WeSCo CBR approach and demonstrate how CBR techniques are applied for semi- automated WS composition. Our proposal is based mainly on two concepts: ontologies on one hand for the description of the concepts and resources needed to improve the discovery of the relevant WS ; and Case Based Reasoning (CBR) on the other hand, for the determination of the required busi- ness process. Our contribution consists of: 1) the definition of an ab- stract level defined by a set of domain activities in order to limit the search space of WS and 2) a method to build an abstract process and then a composite WS through a set of similarity computation procedures according to the CBR techniques.