There are no psychological characteristics or psychopathology that separates terrorists from the general population. Rather it is group dynamics, with a particular emphasis on collective identity that helps explain terrorist psychology. Just as there is a diverse spectrum of terrorisms, so too is there a spectrum of terrorist psychologies. Four waves of terrorism can be distinguished: the Anarchist wave, associated with labor violence in the United States in the late 19th century; the Anti-Colonial wave (nationalist-separatist), with minority groups seeking to be liberated from their colonial masters or from the majority in their country; the New Left wave (social revolutionary); and now the Religious wave. With the communications revolution, a new phenomenon is emerging which may presage a fifth wave: lone wolf terrorists who through the Internet are radicalized and feel they belong to the virtual community of hatred. A typology of lone wolf terrorism is proposed.