Incels are a primarily online sub-culture community of men who forge a sense of identity around their perceived inability to form sexual or romantic relationships. The incel community operates almost exclusively online, providing an outlet to express misogynistic-hostility, frustration and blame toward society for a perceived failure to include them. A small proportion of incels have committed acts of violence against wider society. Incels purport that genetic factors, evolved mate preferences, and inequitable social structures restrict their access to sexual relationships with women. They believe that most women are attracted to a small number of men (Chads), who monopolize sexual encounters, while the “genetically inferior” incels are excluded from the gene pool. Although incel research is still in its infancy, early foundational studies have highlighted extremely poor mental health, some cognitive distortions, a victimhood mindset known as the black pill, a disproportionate prevalence of autism spectrum disorder, poor mating intelligence, and high levels of dating anxiety among incels. This chapter reviews and synthesises the current literature on incels, outlining the historical development of the community, their demographic profile, ideological worldview, and potentiality for harm to themselves and others.