SummarySeptins are a family of proteins highly conserved in eukaryotes that have been under intense scrutiny lately because they appear to comprise a novel component of the cytoskeleton. As such, septins play important roles in many cellular processes by providing rigidity to the cell membrane, serving as scaffolds to recruit proteins to specific subcellular locales, and creating membrane diffusion barriers to establish discrete cellular domains.Septins were first discovered in yeast during screens for genes that are critical for cell division. Orthologs have since been identified in animals, fungi, and algae; but, interestingly, septins are absent from higher plants. The number of septin genes ranges from a minimum of two in Caenorhabditis elegans (UNC-59 and UNC-61) to 14 in humans (SEPT1 through SEPT14). To complicate matters, the transcripts of many mammalian septin genes undergo alternative splicing, making the number of unique septin isoforms even greater. Despite this complexity, all septins bind guanosine-5′-triphosphate (GTP), and interact with each other to form complexes. Septin complexes further associate with each other to form large filamentous structures, as is observed with other cytoskeletal proteins. In this primer, we provide an overview of the structural and functional features of septins, and highlight their involvement in various human diseases.