In American jurisprudence, the public trust doctrine emerged as a means of protecting certain limited environmental interests, such as coastal waterways and fishing areas, which were preserved for the benefit of the public and distinguished from grants of private ownership. However, modern scholars have called for an expansive application of the public trust doctrine, citing the growing inventory of “changing public needs” in the environmental context, such as the need for improved air and water quality, and the conservation of natural landscape. This Article examines the history and scope of the public trust doctrine to determine how modern resource management fits within the doctrine’s development under the Constitution and common law. Such an examination is incomplete without reviewing the important principles of Natural Law underlying the original doctrine. In the end, the Article concludes that modern trust expansion should be limited within the ancient values of principled economic reasoning.