The principles of ijtihad in the Maliki, Hanbali, and Twelver Shia schools of thought reflect differences in their attitudes toward sources and methods of deriving religious rulings. While these three schools share a common reliance on the Quran and Sunnah, they differ in the extent of validity of reason, analogy, consensus, and the practice of the people of Medina. This descriptive-analytical study seeks to answer the basic question of what are the principles of ijtihad in the Maliki, Hanbali, and Twelver Shia schools of thought? The findings indicate that all three schools agree on the principle of deriving from the Quran and Sunnah, but they have fundamental differences in the sources and limits of ijtihad. The Maliki school emphasizes the Sunnah of Medina and the practice of its people as a valid source, the Hanbali school of thought, with a textualist tendency, relies on hadith and the appearance of texts, and does not accept analogy and opinion except when necessary, while the Twelver Shia school of thought, by accepting reason as an independent source alongside the Quran, Sunnah, and consensus, provides a rational and systematic basis for ijtihad. Therefore, each has a specific approach to deriving religious rulings while maintaining a connection with religious texts. These differences reflect the diversity and dynamism of Islamic jurisprudence in responding to the intellectual and social developments of each era.