Traditional passwords have numerous problems, but so far no alternative system has been able to replace them. Authentication using brain-computer interfaces (BCIs), or passthoughts, has been proposed as an alternative because of the unique biometric properties of neurophysiological data, but significant questions remain regarding usability and practicality. In this thesis I explore issues related to the usability of BCIs and passthoughts. I designed and built a prototype passthought authenticator, which revealed significant usability issues surrounding the use of mental commands. By interviewing expert BCI users and researchers, I identified barriers relating to perceived safety, usability, and applicability of BCIs. A survey of MTurk workers with no prior BCI experience revealed that personality characteristics as well as security behaviours were related to respondents' acceptance and perceived usability of BCIs. These studies revealed significant barriers and areas for improvement of passthoughts and BCIs in general.