In plant development, the basic body plan is laid down during embryogenesis. Development carries on postembryogenically above and below ground with the continuous formation and outgrowth of lateral organs shaping the adult plant. In the past two decades, molecular genetics has been the preferred approach to study Arabidopsis thaliana root development. These efforts have resulted in the identification of numerous genes, involved in as many regulatory processes of root growth and development. Incidentally, conserved mechanisms and genetic factors that act in root and shoot growth have been uncovered, revealing general principles of plant development. Transport-mediated graded distribution of the phytohormone auxin, for example, acts as a global organizer that is locally translated into distinct cellular responses by specific auxin/indole-3-acetic acid-AUXIN RESPONSE FACTOR pairs. In the root, these responses promote expression of the PLETHORA regulators that act dose-dependently in controlling root morphology. The stem cell niche is uniquely defined by the combinatorial activity of the PLETHORA and SHORT-ROOT/SCARECROW transcription factors yet deploy signaling mechanisms that are conserved in root and shoot stem cell maintenance. Perpetual divisions of the stem cells are tightly regulated, interconnecting epigenetic factors, hormonal control and core cell cycle components. In this chapter, we will focus on recent advances in our understanding of Arabidopsis root development. Taking embryogenesis as a starting point, we will describe the genes and mechanisms involved in root meristem and stem cell patterning and maintenance.