Lone-actor terrorism is a low-frequency phenomenon. Many people adopt extremist or radical views that endorse the use of violence, but only an infinitesimal minority ever conduct individual acts of violence or terrorism. Moreover, lone-actor terrorists are often thought to prepare and perpetrate violent attacks on their own but also as predominantly ‘self-radicalized’ individuals. How are they socialised into radical worldviews and/or how do they adopt violent forms of action without the support of militant groups and radical milieus? Research in recent years has confirmed that so-called lone actors are in fact not as ‘lone’ as the description seems to suggest. As much as collective radicalisation is a relational process driven by interactions with other individuals, movements, and institutions in one’s immediate social environment, lone-actor radicalisation is similarly relationally informed. The ‘loneness’ of lone actors is always relative and never absolute and is itself a form of relational configuration. Drawing on the Lone Actor Radicalisation and Terrorism (LART) dataset (N=306), this chapter identifies the emergence and development of the field, with a particular focus on the most recent innovations and transformations of lone-actor radicalisation. The chapter makes the case for a relational perspective. We argue that, in addition to examining the social and personal backgrounds and histories of these perpetrators, we need to examine the dynamic social processes in which lone-actor attacks are embedded, as shaped and driven by online as well as offline relations and interactions.