Rheb (Ras homolog enriched in brain) is an evolutionarily conserved member of the Ras family of small GTP binding proteins originally found to be rapidly induced by synaptic activity in the hippocampus following seizure. While it is expressed at relatively high levels in the brain, Rheb is widely expressed in other tissues and may be induced by growth factor stimulation. Similar to other family members, Rheb triggers activation of the Raf-MEK-MAPK pathway. Biochemical and genetic studies demon-strate that Rheb has an important role in regulating the insulin/Target of rapamycin (TOR) signaling pathway. TOR is a serine/threonine protein kinase that acts as a sensor for ATP and amino acids, balancing the availability of nutrients with protein translation and cell growth. A dimeric protein complex termed TSC1/TSC2 indirectly inhibits TOR activity by inhibiting Rheb via the GAP activity of TSC2.