The current political climate is awash with groups that we might be tempted to label irrational, extremist, and hyper-partisan; it is full of echo-chambers, radicalization, and epistemic bubbles. Philosophers have profitably analyzed some of these phenomena. But philosophers have largely overlooked another crucial aspect of our time: the way in which certain groups are fanatical. I distinguish fanatical groups from other types of problematic groups, such as extremist and cultish groups. I argue that a group qualifies as fanatical only if it has features that promote individual fanaticism. But how might a group promote individual fanaticism? I argue that a typical feature of fanatical groups is their tendency to encourage an emotion that philosophers sometimes call “ressentiment,” which differs from ordinary resentment. I explain what ressentiment is, how it can be fostered, and how it can lead to fanaticism. I contend that this account helps us to identify a disturbing and increasingly widespread feature of contemporary social and political groups.