A quantitative study of 130 individuals arrested in Spain between June 2013 and August 2016 for terrorist activities related to the Islamic State (IS) shows that the vast majority of them were involved in jihadist activities together with others and not as lone actors. They were typically part of cells, groups or networks (CGN) of varied size and composition. These CGN were more often new, transnational and IS-linked than regenerated, national and only IS-inspired. Detainees who participated in CGN occupied various positions in the centre (or first-tier), in the intermediate circle (or second-tier), and the periphery, depending on their social characteristics and functions. Nearly all of them belonged to jihadist aggregates engaging in radicalisation and recruitment efforts, usually dispatching foreign fighters, raising money and distributing propaganda on behalf of IS. In addition, the majority of these individuals had either travelled to Syria and Iraq, had tried (but failed) to travel, or had the intention of doing so. About one third of the 130 individuals belonged to CGN with operational capabilities and manifested willingness to carry out attacks inside Spain. Our research on the Spanish situation shows that the threat posed by IS is highly networked and organized.