The focus of the lone-actor terrorism research has been quite wide. Some studies focus upon ideological motivations and lone-actor terrorist psychology, including the role of media, such as online media, in motivational development and ideological change. Some dealt with preventing, countering or responding to lone actors and the attacks they may execute. The chapter summarizes the existing knowledge on lone-actor radicalisation, attack planning and the attacks themselves. In the domain of radicalisation studies, the boundaries between 'types' of radicalisation, on a spectrum of 'lone' to 'group', appear increasingly hazy as more information comes to light on individual cases. The chapter reviews the knowledge-base about the behaviour of lone actors, as it relates to the preparation and the commission of attacks. The synthesis of the research literature on lone-actor terrorism highlights the diversity of factors, indicators, mechanisms, processes, and overall concepts that have been associated, analytically or empirically, with the search for an explanation of lone-actor radicalisation and behaviour.