With its straightforward amenability to genetic approaches, Arabidopsis thaliana proves to be a powerful system to study biological processes in plants. The root of Arabidopsis has several additional features which make it a good model system to study developmental processes. It has a largely invariant ontogeny and fixed cellular organization. Continuous cellular differentiation in the root meristematic region allows monitoring of developmental processes after embryogenesis. The Arabidopsis root is also amenable for experimental manipulation. The simplicity of its organization allows large-scale mutant screens covering several aspects of root formation and growth. Experimental manipulations have uncovered the existence of positional cues important for pattern formation. Together with the several classes of mutants and the ongoing identification of genes these findings contribute to a better understanding of developmental processes at the cellular level.