Custom (ʿurf) is a subsidiary source of Islamic jurisprudence. It plays an important role in regulating human transactions in accordance with the objectives of Islamic law. This study explains the juristic distinction between valid and invalid custom. It first examines the lexical and technical meanings of ʿurf. It then discusses the factors that lead customs to emerge and persist. The study classifies custom as valid when it accords with Islamic law and as invalid when it conflicts with its texts or objectives. It also considers the authority of custom in light of the Qur’an, the Prophetic Sunnah, and practical consensus. The study identifies the conditions and parameters that govern the legal recognition of custom. It then presents the position of the International Islamic Fiqh Academy on moral rights as a contemporary application of custom. The analysis confirms that such rights must remain subject to the established principles of Islamic law.