‘Non-knowledge’ is a classical sociological term introduced into sociology most prominently by Georg Simmel. Dismissing classical sociological concepts as ‘zombie categories’, Ulrich Beck turned to non-knowledge relatively late in his career. This article argues provocatively that many of Beck’s observations on issues ranging from the uninsurability of modern risks to the notion of risk itself would have greatly profited from being complemented or even substituted by a theory of not knowing. Viewed in this light, Beck’s notion of risk no longer applies to the world he describes and thus has become one of his own zombie categories. The article then takes up some of Beck’s unfinished attempts at conceptualizing the unknown and develops them further so that they might prove useful for security studies and related fields today. To illustrate this approach, the article uses examples of non-knowledge on matters ranging from potential terrorist attacks to issues of human health and security. © 2016, © The Author(s) 2016.