In the early stages of a data warehouse project, the designers/administrators have to come up with a decision concerning the design and deployment of the back-stage architecture. The possible options are (a) the usage of a commercial ETL tool, or (b) the development of an in-house ETL prototype. Both cases have advantages and disadvantages. However, in both cases the design and modeling of the ETL workflows have the same characteristics. The scope of this chapter is to indicate the main challenges, issues, and problems concerning the manufacturing of ETL workflows, in order to assist the designers/administrators to decide which solution suits better to their data warehouse project and to help them construct an efficient, robust and evolvable ETL workflow that implements the refreshment of their warehouse.