Recently, our knowledge concerning the role of amino acids in signal transduction in mammals has greatly improved. This significant advance is mainly due to the remarkable discovery that the mammalian target of rapamycin (mTOR) protein kinase, known to be activated in response to a large number of hormones, growth factors and cytokines, is also under the tight control of branched-chain amino acids. Actually, both inputs are necessary to fully activate the mTOR pathway, the main function of which is to increase cell size, via the regulation of translational processes. However, amino acids are able to modulate other biological effects and appear to have unexpected actions, as evidenced by our recent work in rat adipocytes. The aim of this review is to summarize novel findings on the role of mTOR and amino acids in insulin signaling in adipocytes. A possible beneficial impact of the use of amino acids in the treatment of insulin resistance is discussed, and hypotheses about the molecular mechanisms underlying their effect are proposed.