Growth factors and nutrients regulate the mTORC1 [mammalian (or mechanistic) target of rapamycin complex 1] by different mechanisms. The players that link growth factors and mTORC1 activation have been known for several years and mouse models have validated its relevance for human physiology and disease. In contrast with the picture for growth factor signalling, the means by which nutrient availability leads to mTORC1 activation have remained elusive until recently, with the discovery of the Rag GTPases upstream of mTORC1. The Rag GTPases recruit mTORC1 to the outer lysosomal surface, where growth factor signalling and nutrient signalling converge on mTORC1 activation. A mouse model of constitutive RagA activity has revealed qualitative differences between growth-factor- and nutrient-dependent regulation of mTORC1. Regulation of mTORC1 activity by the Rag GTPases in vivo is key for enduring early neonatal starvation, showing its importance for mammalian physiology.