2) the user profile is restricted to the domain of interest dimension and is composed of a set of weighted selection predicates, (3) user queries are conjunctive queries of type Select-Project-Join, and (4) terminological problems between terms (attribute and table names) are supposed to be resolved. The goal of query reformulation is to answer the following questions: (1) How to take into account the multiple data sources? and (2) How to consider the user preferences? To answer these questions, two types of techniques are used: (1) query rewriting which is used to select data sources and to substitute the query variables and (2) query enrichment which exploits the user profile. The query rewriting pro- cess consists in transforming the user query expressed on the virtual schema into expressions defined on the data sources [25]. The query enrichment process exploits the user profile to enhance expressiveness of a given query by integrating specific knowledge taken from the user profile [34]. Our reformulation process consists in four steps (Figure 10.9). The first step (query expansion) consists in expanding the initial query with rel- evant virtual relations. The set of selected virtual relations is chosen according to two criteria: (1) the profile predicates that can be expressed on these relations and (2) the size of the set. The first criterion has to be maximized to allow a good per- sonalization whereas the second has to be minimized to not penalize the rewriting Virtual schema Expanded User query query Relevant sources Query expansion identification Relevant sources User profile Rewritings Relevant sources Final enrichment combination Data source schemas Reformulated user query Figure 10.9 Query reformulation process. Personalization Paradigm in Service Delivery Platforms process too much. The selected relations are then integrated to the initial query. The result of this step is an expanded query on which a sufficient number of profile predicates can be expressed. The second step (relevant sources identif