The root endodermis is the cylindrical boundary that separates the inner vascular tissue from the outer cortex and functions as an apoplasmic barrier for selective nutrient uptake. Recent developmental and cell biological studies have started to reveal the mechanisms by which this single cell layer serves as a key regulatory module of root growth, tissue patterning and nutrient flow, which in concert support the plant's ability to survive in a terrestrial habitat. This review provides an overview of the key factors that contribute to the functioning of the root endodermis and discusses how this single cell layer dictates root growth and tissue patterning.