Insulin-like growth factors (IGFs) appear to play a role in the development of tumors in general and brain tumors in particular. Specific receptors for IGFs have been identified in normal human and rat brain, and evidence suggests that components of the IGF signal transduction system may play a role in the transformation process. Secretion of IGFs by a variety of human brain tumors has been confirmed, and these growth factors appear to have an autocrine stimulatory effect on these tumors. IGFs circulate in the blood stream bound to at least six distinct binding proteins which may modulate the effects of these growth factors on target tissues. Sex steroids may also regulate the behavior of certain brain tumors such as meningiomas at least in part through their effects on the expression of IGFs and their binding proteins. Recently, antisense gene technology against certain IGFs or their receptors have resulted in potent antitumor effects in the case of several gliomas, although the mechanism for this remains unclear.