This thesis is an interdisciplinary project; I develop a new philosophical theory of conscientiousness as a neo-Aristotelian virtue. I then go on to develop two psychometric measures (one general measure and one student-specific measure) to investigate whether there is empirical evidence in support of the theory. In doing so, I contribute to a vastly under researched area of virtue theory ― the science of Aristotelian virtues. As discussed in Chapter Two, there are currently no validated measures of any of the Aristotelian virtues. Thus, this area of research is not only of interest to philosophers; it also contributes to the general and scientific understanding of people’s psychology, specifically, their traits, and how these traits influence important factors related to human flourishing. In this thesis I first explore important theory and research relating to the virtues; this includes both philosophical theories of virtue and the psychological methods used to investigate them. Of particular importance, I also discuss the differences in terminology and approach taken by philosophers and psychologists in their study of traits generally. This then informs my account of the virtue of conscientiousness (or what I refer to as Virtue Conscientiousness), and the related scales of Character Conscientiousness used to measure it. Finally, the methods and results used to assess Character Conscientiousness are presented and discussed. The findings suggest that, as a character trait, conscientiousness consists of two facets: a virtue facet (Virtue Conscientiousness) and an excess facet (Excessive Conscientiousness). This factor structure was replicated between the two scales which were applied to two different samples. Each Facet (in both scales) displayed meaningful relationships with other variables, with Virtue Conscientiousness positively correlating with life satisfaction and with Excessive Conscientiousness positively correlating with depression, anxiety, and stress. Though preliminary, these findings are promising. They support my theory of Character Conscientiousness, and of the claim that conscientiousness is a virtue.