This essay presents an overview of the development of natural law theories in Spain and Portugal in the late nineteenth and the twentieth centuries. There are some shared historical and cultural features that enable the joint treatment of both the Spanish and Portuguese Natural Law theories. Yet, it would certainly be a mistake to assume an undifferenced approach to these two traditions, which have their own history and peculiarities. Consequently, a common treatment is provided for the forging era of these traditions, in which the interchange of ideas and approaches was more intense; while a separate presentation is considered more appropriate for twentieth century theories, where differences are more acute.