In Drosophila melanogaster, sex is determined at the preblastoderm stage via an Xchromosome counting mechanism. During this process embryos that carry two X chromosomes begin to develop as females while embryos with one X start the male developmental program. The Xlinked genes involved in sex determination, also called Xsignal elements (XSEs), are: sisterlessA (sisA), sisterlessB (sisB), unpaired (upd), and runt. These genes are responsible for the transcriptional activation of the master regulatory gene Sexlethal (Sxl). Expression of Sxl is initially accomplished only in females through activation of the establishment promoter SxlPe. Later in development, Sxl is transcribed in both sexes through a maintenance promoter, SxlPm, but functional Sxl protein is only produced in female flies. Since Sxl is at the top of the sex determination cascade, understanding its regulation is key to comprehend the process of sex determination. The experiments in this dissertation were designed to better understand two aspects of the sex determination mechanism: How the protein encoded by XSE element sisA interacts with SxlPe, and how the transition from regulation by SxlPe to regulation by SxlPm occurs. The sisA protein (SisA), as part of the bZIP protein family, is thought to bind to its target as a dimer, but a dimerization partner has not yet been found. This work uses knockouts and germline clones to examine interaction between sisA and three SisA partner candidates, atf4, CG16813, and CG16815. Although the evidence described here suggest that none of the three SisA partner candidates genetically interact with Sis, we cannot rule out the possibility of redundancy between the different candidate proteins. This research unravels the timing and regulation of SxlPm expression. I have shown, contrary to previous thought, that expression of SxlPe and SxlPm overlaps for a brief period. Several of the same proteins that are involved in the regulation of SxlPe, including the XSE sisB, also regulate SxlPm. This sex specific regulation leads to a sexually dimorphic pattern of activation and early expression of SxlPm. A common enhancer region was found to regulate SxlPe as well as SxlPm. These results highlight the importance of the transition between SxlPe and SxlPm for the proper establishment of sex determination and have implications for how the sex determination mechanism evolved.