Dimensional modelling is a conceptual modelling technique developed for designing data warehouse structures. It has become the predominant approach to designing data warehouses in practice and has proven to be highly successful in developing database structures that can be used directly by end users. This paper examines the nature of dimensional modelling and its relationship to traditional Entity Relationship (ER) modelling. It shows that a dimensional model is just a restricted form of an ER model, and that there is quite a straightforward mapping between the two. Understanding the relationship between the two types of models can help to bridge the gap between operational system (OLTP) design and data warehouse (OLAP) design. It also helps to resolve the difficult problem of matching supply (operational data sources) and demand (end user information needs) in data warehouse design. Finally, it results in a more complete dimensional design, which is less dependent on the designer’s ability to choose the “right” dimensions. The paper also reports some preliminary results from empirical testing of the approach.