The initiation of plant lateral organs from the shoot meristem is associated with the formation of boundaries that separate the primordia from surrounding tissue. A distinctive set of cells is present along the boundary, and these 'boundary cells' display characteristic patterns of cell division, morphology and gene expression. A certain class of the NAC transcription factors is important for growth suppression at the boundary, and auxin and microRNAs participate in boundary formation by regulating NAC gene expression. Other factors regulate different aspects of boundary functions, such as the establishment of the border of different cell identities, the initiation of axillary meristems, or the proper development of organs and tissues in adjacent regions.