CD4+ and CD8+ T cells are critical components of the adaptive arm of immune responses. During viral infection, CD8+ T cells utilize their cytotoxic function to kill infected cells and clear the infection. In addition, CD4+ T cells differentiate into either T helper 1 (Th1) or T follicular helper (Tfh) cells, which provide essential help to enhance the efficacy of other response immune cells, including macrophages, CD8+ T cells, and B cells. The transcription factor, T cell factor-1 (TCF1), and its homologue, Lymphoid enhancer-binding factor-1 (LEF1), have critical roles in the development, differentiation, and persistence of both CD4+ and CD8+ T cells. However, the influence of TCF1 and LEF1 on Th1 and Tfh differentiation remains to be examined. Furthermore, due to alternative promoter usage, TCF1 and LEF1 are expressed as both long and short isoforms. The distinct roles of the long and short isoforms of TCF1 in the context of CD4+ and CD8+ T cell responses have not been defined. My studies utilized multiple novel mouse strains to examine the roles of TCF1 and LEF1 in Tfh and Th1 differentiation during viral infection, and the unique requirements of TCF1 long isoforms in CD4+ and CD8+ T cell responses. Specifically, my initial studies characterized a new TCF1 reporter construct (referred to as p45GFP reporter) and used this reporter to address the specific contributions of TCF1 long isoforms to the CD8+ T cell response. Previous studies have abrogated all TCF1 isoforms and shown that in the absence of TCF1, the memory CD8+ T cell population is dramatically impaired and exhibits defective persistence over time. Here, I showed that TCF1 short isoforms are sufficient for the generation of memory CD8+ T cells, however TCF1 long isoforms are important for the maturation of memory CD8+ T cells. Another critical component of pathogen clearance and long-term protection is a productive humoral response, which is optimized by the B cell help provided by Tfh cells. Using the p45GFP reporter, I showed that TCF1 is specifically retained in Tfh cells, but downregulated in Th1 cells. I utilized a huCd2-Cre system to conditionally delete TCF1 and LEF1 in mature T cells. In response to viral infection, TCF1 and LEF1 double-deficient mice showed normal Th1 responses, but severely defective Tfh differentiation and a concomitant impaired B cell response. I further demonstrated that TCF1 promotes Tfh differentiation by directly regulating many Tfh-associated genes. Furthermore, I used the p45GFP reporter to I identified distinct, but critical, roles for both long and short isoforms of TCF1 in driving Tfh differentiation and repressing differentiation toward Th1 or germinal center Tfh cells. Finally, while TCF1 is known to be critical in the formation of memory CD8+ T cells, its impact on memory CD4+ T cell generation has not been assessed. Once again utilizing the p45GFP reporter, my studies identified an important role for TCF1 long isoforms in the survival of both Th1 and Tfh cells through contraction. In the absence of TCF1 long isoforms, the memory CD4+ T cell population is severely reduced. Taken together, my work has demonstrated critical roles for TCF1 during both effector and memory phases of the CD4+ T cell response to viral infection. In summary, TCF1 is crucial for CD4+ T cells to effectively differentiate and provide important help to B cells during viral infection. Moreover, my studies have identified critical and unique roles for long and short isoforms of TCF1. Finally, TCF1 is necessary for optimal formation of memory CD4+ and CD8+ T cells, and thus is an essential component in achieving protective immunological memory after viral infection.