Iranian-Saudi relations have waxed and waned several times since Iran's revolution in 1979. The animosity between the two countries in the 1980s eventually led to diplomatic ties between the two countries being severed in 1988. After three years of diplomatic impasse and frigidity, however, they finally restored their diplomatic relations in March 1991. Despite the problems both countries faced, they enjoyed remarkable cooperation concerning oil in the late years of the 1990s. Moreover, later on April 18 2001, Tehran and Riyadh signed a security agreement that was a turning point in the relationship between the two countries. With respect to these noticeable gains, the present book answers the question why the two countries, despite differences regarding oil price and quota in most years of the 1990s, could eventually achieve economic cooperation regarding oil within OPEC for the first time in 1999. Moreover, it identifies who played important roles in the economic cooperation between the two countries. Finally, the research reveals how Iran and Saudi Arabia arrived at a remarkable security cooperation in 2001 despite the diverse political and religious problems during the 1990s.