The Mu‘tazilah school represents one of the most significant rationalist theological movements in the history of Islamic thought. Emerging in the 8th century CE in Basra, it was pioneered by Wāṣil ibn ‘Aṭā’ as a response to theological debates concerning the status of grave sinners and as an effort to uphold the principles of divine unity and justice through reason. This study aims to examine the historical emergence, major figures, and fundamental doctrines of Mu‘tazilah, known collectively as Uṣūl al-Khamsah (The Five Principles). Employing a qualitative approach with a library research method, the study explores both classical and modern sources on Islamic theology (kalām) and philosophy. The findings indicate that the Mu‘tazilah positioned reason (‘aql) alongside revelation as a primary means for understanding religious teachings. Its five foundational principles include at-Tawḥīd (the Oneness of God), al-‘Adl (Divine Justice), al-Wa‘d wa al-Wa‘īd (Promise and Threat), al-Manzilah bayna al-Manzilatayn (the Intermediate Position), and al-Amr bi al-Ma‘rūf wa an-Nahy ‘an al-Munkar (Commanding Good and Forbidding Evil). Through these doctrines, Mu‘tazilah emphasized human freedom, moral responsibility, and rational interpretation of religious texts. The existence of this school contributed significantly to the development of Islamic theology (‘ilm al-kalām) and philosophy, particularly during the reign of Caliph al-Ma’mūn of the Abbasid dynasty. Although its influence declined with the rise of Ash‘arism, the intellectual legacy of Mu‘tazilah endures within modern Islamic rationalism. Thus, Mu‘tazilah stands not only as a symbol of classical Islamic intellectual awakening but also as enduring evidence that Islam possesses a profound tradition of critical and rational thought.