Phytohormones have been implicated in vascular development in various ways, but their precise function and the extent of their influence is still controversial. Recent results from experimental manipulation of developing organs and Arabidopsis developmental genetics support a role for polar auxin flow in cell axis formation within the vascular system and, interestingly, also in the embryonic establishment of the plant body axis. Vascular responses to auxin transport inhibition indicate patterns of auxin distribution during leaf development and new technologies may enable these predictions to be tested within the near future. Moreover, recently discovered Arabidopsis axialisation mutants seem to identify essential genes that relay auxin signals in vascular development. A first gene in this class, MONOPTEROS (MP) has been cloned and encodes a transcription factor capable of binding to auxin response elements in the control regions of auxin regulated genes. Molecular identification of further axialisation genes may provide access to a mechanistic understanding of plant cell axis formation.