T-cell differentiation is a tightly controlled developmental program observed as the stepwise progression of immature thymocytes through several unique stages characterized by the expression of distinct combinations of cell surface markers. The advancement of thymocytes from one stage to the next requires the successful completion of one or more specific developmental processes, accompanied by the acquisition of receptors, which signal a cell to transit through certain critical checkpoints to a more mature stage. The techniques of transgenesis and germ-line targeting are particularly useful in the study of such complex developmental programs, in that specific players can be manipulated in the context of an otherwise unaltered environment. Such studies have been instrumental in our understanding not only of how a number of thymocyte surface receptors are involved in the developmental program, but also in identifying some of the intracellular signaling mediators that are involved in regulation of transcription factors which are ultimately responsible for orchestrating the differentiated phenotype.