In an attempt to understand Islamic fundamentalism and its links to terrorism, this chapter, describes four theories presented by the author. The first theory states that not all Muslims are fundamentalists. Fundamentalism does not allow for a clear distinction between the political and the religious spheres. There are four different and sometimes adverse currents to fundamentalism: nationalists, conservatives, modernists, and some political groups. The second theory states that not all fundamentalists are terrorists. It is often among the radical fundamentalists where you find support for terrorism. The third theory states that fundamentalists are Muslims. The fourth and final theory states that not all fundamentalists are terrorists, however it is also true that these terrorists are fundamentalists. There are some forms of terrorism that were created within the Islamic world that do not have a fundamentalist matrix, but are linked to such things as the nationalistic movement. It is necessary that it be remembered that not all Muslims are fundamentalists and that not all fundamentalists are terrorists. It also should not be forgotten that fundamentalists are Muslims, and that fundamentalism is an important component of contemporary Islam, and that these terrorists are an integral part of the radical fundamentalist world.