Amino acids are key nutrients for protein synthesis and many metabolic processes. There is compelling evidence that amino acids themselves regulate protein synthesis, degradation and cell growth. Mammalian target of rapamycin complex 1 (mTORC1) plays a central role in cellular growth regulation. Amino acids potently activate mTORC1, however, the mechanism of amino acid signaling is largely unknown. Recent studies have identified Rag small GTPases as key components mediating amino acid signals to mTORC1 activation.