Much of the data we deal with every day is organized hierarchically: file systems, library classification schemes and yellow page categories are salient examples. Business data too, benefits from a hierarchical organization, and indeed the hierarchical data model was quite prevalent thirty years ago. Due to the recently increased importance of X. 500/LDAP directories, which are hierarchical, and the prevalence of aggregation hierarchies in datacubes, there is now renewed interest in the hierarchical organization of data. In this paper, we develop a framework for a modern hierarchical data model, substantially improved from the original version by taking advantage of the lessons learned in the relational database context. We argue that this new hierarchical data model has many benefits with respect to the ubiquitous flat relational data model.