The serum- and nutrient-sensitive protein kinase mTOR (mammalian target of rapamycin) is a master regulator of cell growth and survival. The mechanisms through which nutrients regulate mTOR have been one of the major unanswered questions in the mTOR field. Identification of the Rag (Ras-related GTPase) family of GTPases as mediators of amino acid signalling to mTOR is an important step towards our understanding of this mechanism.