This book provides an overview of the tumultuous relationship between Saudi Arabia and Iran, exploring the rivalry's impact on the Arab and Muslim world. It traces the origins of the conflict back to the Iranian Revolution of 1979, revealing how both countries have sought to expand their influence through cultural and religious domination. The text analyses the rivalry's role in the rise of sectarianism, terrorism, and violence, culminating in the emergence of the Islamic State (ISIS). It also examines the impact of this conflict on the internal politics of countries like Lebanon, Syria, and Pakistan, where both Saudi and Iranian actors have sought to advance their interests through proxy wars and sectarian tensions.