This thesis investigates the role of the Dicer RNA endonuclease in promoting CD4 and CD8 lineage commitment in developing αβ T lymphocytes. Our laboratory has previously shown that Dicer promotes survival of developing thymocytes undergoing antigen receptor induced DNA double strand breaks. During the course of these studies we made the unexpected observation that mice with T cell specific Dicer inactivation and transgenic expression of the pro-survival molecule BCL2 (LckCre;Eμ;BCL2;Dicerflox/flox) contain a population of aberrant CD4+CD8+ peripheral lymphocytes of unknown origin. I have utilized molecular biology, cellular immunology, and complex mouse genetic models to examine the developmental history and fate of these aberrant CD4+CD8+ cells. My results indicate that aberrant peripheral CD4+CD8+ cells arise from impaired co-receptor silencing of either CD4 or CD8 in MHCI or MHCII-restricted T cells, respectively. Initiation of co-receptor silencing is impaired during thymic development, whereas maintenance of co-receptor silencing in mature T cells does not appear to be defective in the absence of Dicer. Aberrant CD4+CD8+ cells from mice expressing either MHCI or MHCII-restricted TCR transgenes exhibit reduced expression of the master transcriptional regulators Runx3 and ThPOK, which promote commitment to the CD8 and CD4 lineages, respectively. Thus, lineage commitment is impaired in the absence of Dicer. Impaired co-receptor silencing was also observed in developing T cells lacking the RNA endonuclease Drosha, suggesting that miRNAs mediate appropriate co-receptor silencing and lineage commitment during αβ T cell development. These results identify a novel role for the miRNA biogenesis machinery in promoting appropriate lineage commitment during αβ T cell development, and indicate that thymic egress and lineage commitment are separable genetic programs during T cell development.