Discussions of mithraea tend to emphasize their uniformity. While it is true that many earlier mithraea do adhere to an established plan, there are a notable number of mithraea dating from the late 3rd c. onward that do not. This article discusses these various atypical mithraea, how such alterations to the standard mithraeum plan might have impacted on Mithraic rituals, and how this might have affected the experiences of the participants. It also explores why such changes occurred, observing that while in some instances this may have been to accommodate alterations to ritual practices, in others it was likely due to more mundane issues, such as limitations on space and environmental factors. The article concludes by reflecting on the implications this has for the identification of mithraea in the archaeological record.