Monotheism (tawḥīd)-as attested to by the cosmos, known through reason, explicated in revelation, and exemplified by the lives of the righteous-forms the core of the Islamic worldview. A conviction in this unadulterated monotheism unifies Muslims across time and place; it is found in the core profession of faith (the shahāda) and is reinforced by thousands of Qur'anic verses and prophetic teachings. Drawing on the Qur'anic discourse, sayings of the Prophet Muhammad, and select theological works, this Element provides a concise and accessible introduction to the most fundamental concept in Islamic thought. The work explains the nature and attributes of God and examines how tawḥīd informs conceptions of truth, morality, piety, and virtue.