The modern approach to the development of decision support systems (DSS) typically makes extensive use of integrated repositories of data known as a data warehouse. These are often large, complex systems that have unique design and operational requirements that are significantly different to other kinds of databases, despite generally using the same relational database technologies as other systems. This chapter provides an overview of the purpose and design requirements of data warehouses, including approaches to data modeling and structuring of the technical architecture, as well as current trends and practices in the field.