Id proteins are general inhibitors of differentiation and positive regulators of proliferation in the nervous system. The role of Id is especially evident in neural development, a period during which Id proteins prevent premature differentiation and terminal cell cycle arrest. These activities have been related to the recognized ability of Id proteins to antagonize basic helix loop helix transcription factors and proteins of the Retinoblastoma tumor suppressor family. In recent experiments, it was shown that Id proteins-normally low or absent in post-natal tissues-are abnormally expressed by tumor and endothelial cells of neoplasms derived from the central nervous system and peripheral nervous system. In these tumors, deregulated Id activity has been associated with relentless proliferation, loss of differentiation and neo-angiogenesis, all key features of neural tumor progression. Future work will be required to fully elucidate the molecular networks engaged by Id proteins to contribute to the essential hallmarks of neural cancer.