The secretion of interleukin 2 (IL-2) is a key event in T cell activation. IL-2 allows T cells to enter into the S phase of the cell cycle and divide. After the activation phase takes place, T lymphocytes proliferate and differentiate to generate effector T cells. Thereby, T helper (Th) precursor cells, which are functionally immature, may become Th1 or Th2 effector cells. These subsets are responsible for cell-mediated immunity and humoral responses, respectively. Both T cell activation and Th differentiation are processes that depend on changes in the pattern of gene expression. The expression and changes in the genes responsible for these events are regulated by transcription factors. This review will focus on both the transcription factors involved in the control of IL-2 as well as those that are key to T helper differentiation.