This article analyzes the scientific perspectives for studying the frontiers of the Spanish Monarchy at its margins —or epidermis— and in its interior. It questions whether traditional historiographical conceptions offer sufficiently complex visions of the nature and the confines of said monarchy. For this reason, it proposes an interdisciplinary and comparative analysis to explain the historical polysemy of the frontier in the Spanish imperial structure, taking into account cartographical, legal, political, economic, social, mental, cultural, religious, ethnic and emotional elements. All of this leads it to conclude that the internal and epidermal frontier spaces of the empire formed part of its complex nature and directly affected its constitution, dynamism, and dissolution.