This article discusses the ways scholars have outlined the process of Jewish adaptation (or lack of it) from their Christian surroundings in northern Europe during the High Middle Ages. Using the example of penitential fasting, the first two sections of the article describe medieval Jewish practices and some of the approaches that have been used to explain the similarity between medieval Jewish and contemporary Christian customs. The last two sections of the article suggest that in addition to looking for texts that connect between Jewish and Christian thought and beliefs behind these customs, it is useful to examine what medieval Jews and Christians saw of each other's customs living in close urban quarters. Finally, the article suggests that when shaping medieval Jewish and Christian identity, the differences emphasized in shared everyday actions and visible practice were no less important than theological distinctions. As part of the discussion throughout the article, the terminology used by scholars to describe the process of Jewish appropriation from the local surroundings is described, focusing on terms such as “influence” and “inward acculturation,” as well as “appropriation.”