Septins are a conserved family of cytoskeletal proteins implicated in a wide array of cellular functions including cell cycle regulation, ciliogenesis, and cell motility. Two of the most basic functions of septins seems to be compartmentalization of membrane structures and protein scaffolding. Of the four cytoskeletal protein families that provide structure and organization to the cellular environment, septins are the most recently discovered and the least characterized. Like tubulin and actin, septins bind nucleotide (GTP/GDP) and assemble into filaments via interactions between nucleotide-binding domains. Like the intermediate filaments, septins contain coiled-coils essential for the formation of higher-order assemblies. What sets septins apart is their striking structural plasticity. Septin subunits form apolar heteromeric complexes, which self-assemble into filaments, paired filaments, rings, and gauzes. My work has sought to answer three questions about septin plasticity and protein scaffolding. How are distinct septin subunits arranged in heteromeric complexes? How does the arrangement and composition of subunits relate to distinct assembly states and cellular functions? How do the scaffolding properties of septins regulate cellular processes? Using electron microscopy and biochemical techniques to study the structure and function of the mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12 and Shs1) in the budding yeast Saccharomyces cerevisiae, I have found that Shs1 substitutes for Cdc11 at the terminal position in septin octamers and promotes the formation of septin rings, instead of the paired filaments formed by Cdc11-capped octamers. The rings observed in vitro are similar in size as those found in vivo, and Shs1 is essential for the robust formation of the ring-like septin collar at the site of cytokinesis. In addition to subunit exchange, I provide evidence that phosphorylation regulates septin self-assembly. Different phosphomimetic mutations of Shs1 either prevent ring assembly or promote the formation of an alternate assembly, septin gauzes. The implications of these results are discussed. I also studied the molecular organization and self-assembly properties of Spr3 and Spr28, two septins expressed specifically during meiosis and sporulation, an alternate developmental pathway induced by nutrient starvation. I also examined the mechanism by which septins serve as a protein scaffold for the cell-cycle regulator Hsl1, an AMPK protein kinase that appears to act as sensor for septin assembly.