Cytokinesis is a fundamental step of cell proliferation by which daughter cells acquire equal amounts of genetic materials and cellular components. Cytokinesis is precisely regulated in a temporal and spatial manner to ensure that cytokinesis does not occur until chromosome segregation is complete. Failed or precocious cell division causes aneuploidy and/or polyploidy, which is often associated with cancer. In order to coordinate cytokinesis with mitosis, signaling networks have evolved in eukaryotic organisms to faithfully control late cell cycle progression by triggering cytokinesis once mitotic events have been successfully accomplished. In the fission yeast Schizosaccharomyces pombe, this conserved signaling network is known as the septation initiation network (SIN), which triggers actomyosin ring constriction and septum formation after chromosome segregation. The key output of the SIN is thought to be Sid2p kinase activity because Sid2p kinase is the most downstream component of the SIN identified so far, and in addition to the spindle pole bodies Sid2p also localizes to the division site at the end of anaphase, suggesting that Sid2p kinase transmits the division signal from the SPB to the division site, thereby triggering actomyosin ring constriction and septum formation. However, how Sid2p kinase activity is regulated during the cell cycle is still unclear. The goal of this thesis is to understand how Sid2p kinase is regulated. We identified and characterized Mob1p as a novel component of the SIN and a binding partner of the Sid2p kinase. We found that Mob1p is an essential regulatory component important for Sid2p kinase function. Furthermore, we found that phosphorylation is essential for activation of Sid2p kinase and that self-association is able to antagonize Sid2p kinase activity. Thus, we conclude that Sid2p kinase may utilize multiple modes of regulation, including Mob1p binding, stimulatory phosphorylation, and self-association, to control initiation of cytokinesis. Considering the conservation of Mob1p and Sid2p families in the eukaryotes, it is likely that other eukaryotic organisms utilize similar mechanism(s) to control cytokinesis.