Terrorist groups operate under conditions of secrecy. The secret represents both a constraint and an opportunity. Surprisingly, scholars have paid little attention to the study of the organisation of secrecy in terrorism. This article aims to explore this relevant subject. It is divided into four sections. Following Simmel's classic remarks, the first section shows that terrorist groups can be interpreted as forms of secret society. The second section examines the effect of secrecy on the internal organisation of terrorist groups, with particular reference to the management of organisational dilemmas. The third section analyses the secrecy/visibility trade-off. The conclusion outlines some distinctive characteristics of terrorist groups compared to other forms of secret societies, such as secret services and crime enterprises.