The regulation of tissue growth is critically important during embryonic and post-embryonic development across species, and adult tissues must continually be renewed through the tightly regulated proliferation of stem cell populations. In zebrafish, the ventral forebrain and pituitary continue to grow throughout life, making it an excellent system to study the mechanisms that regulate stem cell growth and renewal. We are studying how the cell-cell signaling molecule Sonic Hedgehog regulates cell proliferation during the massive growth phase that occurs during larval development. We have evidence that Shh signaling acts on adult hypothalamic and pituitary stem cells, similar to its role in regulating other neural stem cells. Misregulation of Shh signaling in the adult is associated with human pituitary adenomas, and our studies thus promise to shed light on the mechanisms underlying these common tumors.