Tawhid is the belief that there is only one God. It is the most basic and central belief, and the most important slogan, in Islam. The first doctrine that the Prophet (s) taught people was monotheism expressed with the slogan: "La Ilah Illa Allah" . All ideological, moral and jurisprudential (fiqh) doctrines of Islam go back to Tawhid. Every day in their Adhan (the call for prayers) and prayers, Muslims testify the unity of God or monotheism.

In theoretical monotheism, the belief encompasses all features and aspects of God. God has a unique essence in that there is no being similar to Him (essential monotheism or al-Tawhid al-Dhati , and He is not in need of anyone else while other beings are in need of Him (practical monotheism or al-Tawhid al-Af'ali . This implies that it is only God who is worthy of worship and Muslims should perform their religious practices only for God (worship monotheism or al-Tawhid al-'Ibadi . For Shiites, even God's attributes are nothing over and above His essence (attribute monotheism or al-Tawhid al-Sifati 

Many verses of the Quran have referred to monotheism and God's place. According to the Quran, monotheism is rooted in people's nature (Fitra), all prophets called for monotheism and mostly concerned themselves with the fight against polytheism (Shirk) their goal was to resist polytheism and spread the worship of the only God.