Qadr means measure, or decree. It refers to Allah's complete foreordaining of everything that happens, known and willed by Him before it comes to be. Belief in Qadr is the sixth and final article of faith named in the hadith of Jibril. In the English translation in Sahih Muslim, number ten, faith includes “that you believe in Qadr (Divine Decree) in all its entirety.” And in Surah Al-Qamar, chapter fifty-four, verse forty-nine, an English translation of the meaning reads: “Indeed, all things We created with predestination.”

Scholars have traditionally explained belief in Qadr as resting on four connected levels. The first is Allah's knowledge: His complete knowledge of everything that will happen, before it happens. The second is His writing: Allah's recording of everything that will happen, traditionally associated with what is called the Preserved Tablet. The third is His will: nothing occurs in creation unless He has willed it. And the fourth is His creation: Allah creates all things, including human actions themselves, since nothing exists or happens outside His creating it.

A question raised early in Islamic theology, and still carefully taught today, is how this relates to human responsibility. Belief in Qadr does not mean a person has no real choice, or is not accountable. The Qur'an repeatedly addresses people as responsible moral agents, who will be judged for what they choose to do.

Historically, mainstream Sunni scholarship identified and rejected two opposite errors. One extreme, associated with a group called the Jabriyyah, treated people as having no real choice at all. The other extreme, associated with a group called the Qadariyyah, denied that Allah's prior knowledge and decree extend to human actions. The mainstream position of Ahl al-Sunnah holds both truths together: Allah knows, wills and creates all that occurs, and people genuinely choose their actions and are answerable for them.

Belief in Qadr is often described as bringing contentment rather than fatalism: trusting that what reaches a person could not have missed them, and what misses them could not have reached them, while still striving, working, and making choices, as one is instructed to do.