One of the most important tasks performed in the early stages of a data warehouse project is the analysis of the structure and content of the existing data sources and their intentional mapping to a common data model. Establishing the appropriate mappings between the attributes of the data sources and the attributes of the data warehouse tables is critical in specifying the required transformations in an ETL workflow. The selected data model should besuitable for facilitating the redefinition and revision efforts, typically occurring during the early phases of a data warehouse project, and serve as the means of communication between the involved parties. In this paper, we argue that ontologies constitute a very suitable model for this purpose and show how the usage of ontologies can enable a high degree of automation regarding the construction of an ETL design.