To evaluate concepts, determinants, and health effects of vegetarian diets, a unified, clear, and operational definition is needed. Vegetarianism refers to the exclusion of meat, fish, seafood, and possibly other animal products such as dairy and eggs. A variety of combinations of avoidance of animal products exists. There is no congruence in the scientific literature on how to define and measure different types and strictness of vegetarianism, and self-definitions by vegetarians are even more diverse. This inconsistency in definition and operationalization of vegetarianism leads to difficulties in interpreting scientific publications on health effects of vegetarianism. In this chapter, we will provide a set of definitions as a contribution for a more consistent approach and we will highlight a few classical pitfalls in interpreting data on health effects of vegetarian diets.