Accounts of sectarianism in the current Syrian uprising, offered by participants and observers alike, have a schizophrenic quality: sectarianism seems to be everywhere and nowhere at the same time. Participants challenging the state assert that the uprising began as a civic revolution and was spoiled by the sectarianism of the Syrian regime. The regime and its allies, by contrast, point out the official secularism of the ruling Ba’th party and point to sectarian statements and behaviors of their opponents. Academic accounts generally argue that one of several factors, such as economic change (Hokayem 2013), environmental stress (De Châtel 2014) or spatial inequality (Barout 2012), impelled initial societal mobilization and that the state introduced sectarianism to control the uprising. These accounts rightly highlight some of the ways to which the state has put sectarian boundaries to work, but offer little insight into why they were used in some areas rather than others, at some times rather than others, or the reasons that these techniques resonated with the segments of society in which they were used. Furthermore, extant academic accounts offer little help in disentangling sectarianism from regional, tribal and class solidarities driving actors' behavior in the uprising. This paper argues that the omnipresent but elusive character of sectarianism in Syria stems, in large part, from the phenomenon being poorly defined; it proposes a definition of sectarianism that analytically separates strategies of categorization from networks of actors that crisscross sectarian boundaries. Sectarianism, on this definition, is the invocation of the sectarian boundary to achieve an end an actor could not achieve through networks alone. This definition builds on recent research on social boundary processes; group membership is not an essential property of an individual (something he or she ‘has’) but a relationship constituted by the boundaries that demarcate groups and the networks that bind its members (Brubaker 2004, Tilly 1998, Wimmer 2013). Understood in this way we see, paradoxically, that sectarian boundaries are omnipresent yet invoked relatively infrequently because doing so is costly in terms of limiting future cooperation using networks crossing sectarian boundaries. The paper draws on interview material from recent field research on the first eighteen months of the Syrian uprising to illustrate the concept. It draws, in particular, on events in Homs and the Hassakeh governorate to demonstrate the state’s use of categorization strategies and the factors within society that make this activation and exploitation of boundaries possible.