Data warehouses (DW) appeared first in industry in the mid 1980s. When their impact on businesses and database practices became clear, a flurry or research took place in academia in the late 1980s and 1990s. However, the concept of DW still remains rooted on its practical origins. This entry describes the basic concepts behind a DW while keeping the discussion at an intuitive level. The entry is meant as an overview to complement more focused and detailed entries, and it assumes only familiarity with the relational data model and relational databases.