The postembryonic development of plants is fueled by apical meristems, which are the local production sites of new cells that form a pattern of different cell types within an organ. The regularity of this pattern in the root yielded ideas on its formation from the meristem well before critical studies on the shoot apex were performed (e.g. Hanstein, 1870). The model plant Arabidopsis thaliana, which allows genetic dissection of root development, is a paragon of this regularity. In this Llpdate, we review recent studies on root anatomy and genetics that are allowing us to refine, and perhaps redefine, our understanding of organ development. We will focus on two pivotal aspects of root development: pattern formation and cell proliferation. Important work on other aspects of Arabidopsis roots, such as cell elongation/ morphogenesis, tropism, and cell size and shape, has been covered in recent reviews (Benfey and Schiefelbein, 1994; Dolan and Roberts, 1995) and will not be discussed here.