The control of gene expression at the G1/S transition point (Start) of the cell cycle has been examined using the budding yeast Saccharomyces cerevisae as a model eukaryote. I have shown that the Swi6 protein is the primary activator of the SBF and MBF transcription factors which function at this stage of the cell cycle. The transcriptional activity of Swi6 fluctuates in a cell cycle dependent manner, peaking at the G1/S transition point, around the time of expression of two G1 cyclins, Cln1 and Cln2, as well as other genes under the control of SBF and MBF. Furthermore, the transcriptional activation properties of Swi6 are dependent upon the G1 cyclin Cln3 with which Swi6 interacts in vivo. At the cellular level, Cln3 activity at Start requires the presence of Swi6. Together this thesis demonstrates the pivotal roles the transcription factor, Swi6, and the G1 cyclin, Cln3, play in regulating gene transcription at Start.