Why do some terrorist groups survive considerably longer than others? The literature is just beginning to address this important question in a systematic manner. Additionally, and as with most studies of terrorism, longevity studies have ignored the possibility of interactions between terrorist groups. This article attempts to address these two gaps in the literature: the incomplete understanding of terrorist group survival and the tendency to assume that terrorist groups act independently. In spite of risks associated with cooperation, I argue that it should help involved terrorist groups mitigate mobilization concerns. More importantly, the impact of cooperation is conditioned by attributes of the country in which a terrorist group operates. Using new global data on terrorist groups between 1987 and 2005, I show that cooperation has the strongest effect on longevity in states where groups should have a harder time operating—more capable states and less democratic states. Interestingly, a group's number of relationships is more important than to whom the group is connected.