Septins are a novel family of GTP-binding proteins which are essential for cytokinesis, vesicle trafficking, cytoskeletal reorganization and membrane dynamics. They are abundantly expressed in many mitotic cells. Interestingly, they are also expressed in non-dividing cells such as neurons and platelets in which they play an important role in exocytosis. Platelets from SEPT5 knockout mice show an enhanced serotonin secretion and platelet aggregation in response to subthreshold levels of agonists. Septins are associated with a wide array of critical biological events such as neoplasia, neurodegenerative diseases, infections and exocytosis. The role of septins in oncogenesis is complex. Increased expression of some septins seems to trigger the growth of tumor cells. However, other septin isoforms are shown to promote apoptosis and function as tumor suppressor proteins. Interestingly, septins form complexes consisting of multiple septin polypeptides and assemble into filaments and ring-like, higher-order structures. The different septins and their various isoforms seem to determine the function of the septin complex.