This thesis investigates "catanionic" vesicles, which are nanoscale containers that are spontaneously formed by mixing cationic and anionic surfactants. These structures are easy to prepare and indefinitely stable. In comparison, unilamellar liposomes based on phospholipids are cumbersome to prepare, requiring multiple steps and intense shear (extrusion or sonication); moreover, they have limited stability, especially when stored at room temperature. Despite the many advantages, catanionic vesicles are not frequently used in the pharmaceutical industry because of concerns over their cytotoxicity. In this thesis, we systematically explore the cytotoxicity (on mammalian cell lines) of a range of catanionic vesicles formed by mixing various commercially available cationic and anionic surfactants. We examine how cytotoxicity is influenced by the surfactant tail length, the nature of the surfactant tail (saturated vs. unsaturated) and the net charge on the vesicles; as a control, we also study liposomes from phospholipids. A live/dead assay was our primary tool for assessing cytotoxicity. Our results reveal several systematic trends and we have found that certain vesicles based on unsaturated cationic surfactants are relatively nontoxic and biocompatible. These results could potentially lead to new classes of catanionic vesicles that could be safely utilized for biomedical applications.