This treatise aims to make a study of the law on citizenship in the Philippines as embodied in Article IV, Section 1 of the Constitution of the Philippines with the main objective of determining who are citizens of the Philippines. In analyzing said law on citizenship, the writer has considered decisions of the Supreme Court and the Court of Appeals of the Philippines, opinions of the Secretary of Justice, decisions of American tribunals on the question of citizenship, proceedings of the Philippine Constitutional Convention during the discussions on the article on citizenship, treatises on international law, and other Philippine and American textbooks on citizenship. An adequate exposition of who are citizens of the Philippines, within the limited scope of this treatise, has made the writer limit himself to Article IV, Section 1 of the Constitution, the author making only brief observations on the loss and reacquisition of Philippine citizenship.