THIS STUDY PROPOSES an anatomically based nomenclature for the internal carotid artery (ICA) that can be applied by all disciplines. In 1938, Fischer published a seminal paper describing five segments of the ICA that were designated C1 through C5. These segments were based on the angiographic course of the intracranial ICA rather than its arterial branches or anatomic compartments. Subsequent attempts to apply modern nomenclature to these numerical segments failed to recognize Fischer's original intent of describing patterns of arterial displacement by tumors and, therefore, resulted in a nomenclature that was anatomically inaccurate. Fischer's system was further limited, because segments were numbered opposite the direction of blood flow and the extracranial ICA was excluded. The authors propose a new classification, which includes the entire ICA, uses a numerical scale in the direction of blood flow, and describes the segments of the ICA according to a detailed understanding of the anatomy surrounding the ICA and the compartments through which it travels. Twenty cadaveric specimens with intravascular injection of silicone rubber were used for microscopic dissection and 20 dry skulls were inspected. Histological sections in critical areas were examined. The authors' classification has the following seven segments: C1, cervical; C2, petrous; C3, lacerum; C4, cavernous; C5, clinoid; C6, ophthalmic; and C7, communicating. This classification is practical, accounts for new anatomic information and clinical interests, and clarifies all segments of the ICA.