Various psychological factors have been found to be associated with increased tendencies to identify as vegan or vegetarian. In this chapter, we review some of the dominant psychological factors associated with those tendencies. For example, it is widely established that members of some demographic categories (e.g., women) are more likely to identify as vegan or vegetarian than other members (e.g., men). But an outstanding question is why? What are the psychological factors that are responsible for differences in vegan or vegetarianism identity as a function of gender? Our review aims to help answer these questions based on an already existing and sizable psychological literature. These factors largely fall into three major categories: non-cognitive states (e.g., affect/attitudes), cognitive states (e.g., knowledge), and general cognitive abilities (e.g., deliberative thinking styles). Given the currently existing work, we highlight some areas of strength (e.g., values and attitudes) and also identify some opportunities for future research (e.g., vegetarian or vegan identity as a function of knowledge). We also review some of the problematic issues in measurement of vegan and vegetarian identities including misunderstandings of what veganism is. Finally, we review some relatively new data about the effectiveness of interventions on psychological factors related to vegan or vegetarian identity.