The Tec family tyrosine kinase, Itk, plays an important role in signal transduction following T cell receptor engagement. Several prior studies have established the importance of Itk in immune system processes, such as T cell development and T cell activation. Additional biochemical studies have found that Itk specifically functions within a multi-molecular signalosome complex, which ultimately functions to provide a platform by which Itk can phosphorylate and activate PLC-γ1, a crucial step in T cell activation. To further study how Itk regulates distinct immune outcomes via T cell effector processes within the peripheral immune system, and to further understand how Itk functions in T cells in response to a physiological ligand-receptor interaction, I crossed Itk-deficient mice to mice transgenic for a TCR specific for a moth cytochrome C peptide. My studies have established a unique role for Itk in several important aspects of T cell function. Following T cell activation, I identified an imperative role for Itk in activation-induced cell death via FasL, a mechanism of immune homeostasis. Furthermore, I found Itk plays a unique role in the process of T cell differentiation, where Itk positively regulates the induction of cytokine genes, such as IL-4, while negatively regulating the induction of T-bet, a transcription factor important for Th1 differentiation. Lastly, following T cell differentiation, I found that Itk mRNA and protein are up-regulated during Th2 differentiation, while Rlk, a related Tec kinase, disappears rapidly from Th2 cells, indicating a critical role for Itk in Th2 cell function. Collectively, my thesis work has more clearly defined an important function for Itk not only in TCR signaling, but also in immune processes such as T cell differentiation and activation-induced cell death that are required for proper immune function.