All aboveground organs of plants, including stems, leaves, branches, and flowers, are ultimately derived from a small pool of cells called the shoot apical meristem (SAM). Located at the tip of the growing stem, the SAM contains a pool of pluripotent stem cells that differ from animal stem cells owing to their intrinsic ability to sustain indeterminate growth throughout the life cycle of the plant. This unique property of the SAM to maintain embryolike growth during adult stages accounts for a major difference in developmental strategies between animals and plants. Whereas most animals cease organogenesis (organ formation) quite early in development, plants continue to grow via the addition of newly developed stems and leaves on top of stems and leaves formed earlier in development. As a result, the SAM must maintain a precise equilibrium whereby cells lost during organogenesis are replenished by stem cells that divide to maintain the SAM and furnish new cells toward the formation of additional organs. Thus, the SAM performs two essential functions, namely (1) organogenesis and (2) self-maintenance. Research on SAM biology is focused on understanding the genetic and biochemical parameters controlling these two fundamental components of SAM function.