A wali (wali Arabic: وَلِيّ, walīy; plural أَوْلِيَاء, ʾawliyāʾ), the Arabic word which has been variously translated "master", "authority", "custodian", "protector", is most commonly used by Muslims to indicate an Islamic saint, otherwise referred to by the more literal "friend of God".

When the Arabic definite article al (ال) is added, it refers to one of the names of God in Islam, Allah – al-Walī (الْوليّ), meaning "the Helper, Friend".

In the traditional Islamic understanding of saints, the saint is portrayed as someone "marked by [special] divine favor ... [and] holiness", and who is specifically "chosen by God and endowed with exceptional gifts, such as the ability to work miracles". The doctrine of saints was articulated by Muslim scholars very early on in Islamic history, and particular verses of the Quran and certain hadith were interpreted by early Muslim thinkers as "documentary evidence" of the existence of saints. Graves of saints around the Muslim world became centers of pilgrimage – especially after 1200 CE – for masses of Muslims seeking their barakah (blessing).

Since the first Muslim hagiographies were written during the period when the Islamic mystical trend of Sufism began its rapid expansion, many of the figures who later came to be regarded as the major saints in orthodox Sunni Islam were the early Sufi mystics, like Hasan of Basra (d. 728), Farqad Sabakhi (d. 729), Dawud Tai (d. 777–781), Rabia of Basra (d. 801), Maruf Karkhi (d. 815), and Junayd of Baghdad (d. 910). From the twelfth to the fourteenth century, "the general veneration of saints, among both people and sovereigns, reached its definitive form with the organization of Sufism ... into orders or brotherhoods". In the common expressions of Islamic piety of this period, the saint was understood to be "a contemplative whose state of spiritual perfection ... [found] permanent expression in the teaching bequeathed to his disciples". In many prominent Sunni creeds of the time, such as the famous Creed of Tahawi (c. 900) and the Creed of Nasafi (c. 1000), a belief in the existence and miracles of saints was presented as "a requirement" for being an orthodox Muslim believer.

Aside from the Sufis, the preeminent saints in traditional Islamic piety are the Companions of the Prophet, their Successors, and the Successors of the Successors. Additionally, the prophets and messengers in Islam are also believed to be saints by definition, although they are rarely referred to as such, in order to prevent confusion between them and ordinary saints; as the prophets are exalted by Muslims as the greatest of all humanity, it is a general tenet of Sunni belief that a single prophet is greater than all the regular saints put together. In short, it is believed that "every prophet is a saint, but not every saint is a prophet".

In the modern world, the traditional Sunni and Shia idea of saints has been challenged by puritanical and revivalist Islamic movements such as the Salafi movement, Wahhabism, and Islamic Modernism, all three of which have, to a greater or lesser degree, "formed a front against the veneration and theory of saints". As has been noted by scholars, the development of these movements has indirectly led to a trend amongst some mainstream Muslims to resist "acknowledging the existence of Muslim saints altogether or ... [to view] their presence and veneration as unacceptable deviations". However, despite the presence of these opposing streams of thought, the classical doctrine of saint veneration continues to thrive in many parts of the Islamic world today, playing a vital role in daily expressions of piety among vast segments of Muslim populations in Muslim countries like Pakistan, Bangladesh, Egypt, Turkey, Senegal, Iraq, Iran, Algeria, Tunisia, Indonesia, Malaysia, and Morocco, as well as in countries with substantial Islamic populations like India, China, Russia, and the Balkans.