Nitrogen (N), one of the most important nutrients for plants, is an important signal regulating plant growth and development. Four main morphological adaptations of the root system in response to the N supply have been reviewed: (1) A localized stimulatory effect of external nitrate on lateral root elongation. The AtNRT1.1 (CHL1) dual-affinity nitrate transporter acts upstream of the ANR1 MADS box gene to mediate this stimulatory effect. (2) A systemic inhibitory effect of high tissue nitrate concentrations on the activation of lateral root meristems. The RNA-binding protein, FCA, could be a component of a signaling transduction pathway involved in the high nitrate/ABA induced inhibition. (3) A suppression of lateral root initiation by high C∶N ratios. The AtNRT2.1 high-affinity nitrate transporter seems to be involved in this repression. (4) An inhibition of primary root growth and a stimulation of root branching by external L-glutamate. This morphological response of the root system probably involves sensing by plant homologues of mammalian ionotropic glutamate receptors (iGluRs). The root morphological responses to the N supply have important physiological and ecological significance.