Lymphocyte development is a process in which proliferation is coupled to differentiation. In order to undergo efficient proliferation, lymphocytes must coordinate the entry into cell cycle with increased metabolism. The signaling pathways, like those downstream of antigen and cytokine receptors, and specific regulators that directly control cell metabolism are only beginning to be defined. A study in this issue of the European Journal of Immunology, demonstrates that the liver kinase B1 (LKB1) is a regulator of cellular metabolism. Deficiency in LKB1 results in a block in T-cell differentiation, increased apoptosis, as well as impaired survival of mature T cells. This study highlights the importance of LKB1 in T-cell development and function, but as discussed in this Commentary, a number of intriguing questions concerning the regulation and additional functions of LKB1 remain.