Lone actor terrorists of various ideological hues provide the predominant terrorist threat in the West today. Empirical research on the topic has involved both qualitative and quantitative research: open and closed source data and research on both the offenders and the offences themselves. The purpose of this paper is to synthesise these findings. Broadly speaking, the literature has come to eight key conclusions. Firstly, there is no socio-demographic profile. Secondly, generally somebody knew something in the build-up about the individual’s motivations, planning and intent. Thirdly, motivation does not centre purely around ideology. Fourthly, the attacks are rarely sudden and impulsive but things may be changing. Fifthly, lone-actor terrorists look and behave a lot like mass murderers. Sixthly, mental health problems are common and complex within the offender sample. Seventhly, “online radicalisation” is a misnomer. Eighthly, managing the problem will entail interdisciplinary and multi-agency approaches. The paper concludes with a brief exploration of where research and practice needs to go next.