It is important to know the anatomy of the parenchymal brain and cerebral vascular landmarks (circle of Willis) for an adequate US approach and interpretation of the findings in the critical patient bedside. 2. Within the circle of Willis the most frequent anatomical variation is hypoplasia or absence of the posterior communicating artery (PComA). 3. It is paramount to choose the most appropriate bone window according to the clinical condition of the patient, his decubitus, and the specific objective to be studied. 4. The transtemporal acoustic window is the most used because it allows access to the most important axial exploration planes: mesencephalic, diencephalic, and ventricular. 5. The submandibular acoustic window is an alternative and very useful access, especially in the critically ill patient and his frequent supine decubitus, when studying the vertebrobasilar system. 6. The anatomical identification of the deep venous cerebral system and the dural venous sinuses is not easy. It requires training and dedication to the bedside of the patient.