The kidney, like other organs, grows in constant proportion to the rest of the body. When one kidney is removed, the remaining one hypertrophies. In a comprehensive series of studies, Chen et al. show that growth during maturation is mediated by the mTORC1 signaling pathway, which is induced by EGF-like peptides, and requires PI3K, PDK, AKT, mTORC2, and activation of mTORC1 through the combined effects of TSC and RHEB as part of a multiprotein complex localized on lysosomes. However, compensatory growth is mediated by amino acids, which act on mTORC1 independently of the previous pathway, and requires a class III PI3K (VPS34) that is known to be involved in vesicle trafficking to the lysosomes.