This article presents two distinct modes of operating in a state of clandestinity adopted by Italian leftist terrorist groups, such as the Red Brigades and First Line, in the second half of the 20th century. The two modes of clandestine life are specified with the terms “invisibility” and “camouflage”. The invisibility mode of clandestinity imposes a regime of “secret life” on the group members, while the camouflage mode of clandestinity imposes a “double life” regime on them. The research aims to construct two simplified models, or, to use the Weberian terminology, two “ideal types”. Our primary sources are autobiographies published by former terrorists, official propaganda documents and pamphlets compiled by terrorist groups, and court rulings. Our secondary sources are journalist reports and research published by experts in political violence. From the theoretical point of view, the conclusion is, that for law enforcement, it is much more difficult to combat terrorist formations imposing the double life regime on their members rather than a secret life regime. Still, the double life regime is more stressful from a psychological point of view, as it requires an artificial split of personality. In the conclusions, the article expands the discussion to non-Italian terrorist organizations, with a different political or religious agenda.