Purpose of Review <p>The review aims to summarize postoperative disturbances in cerebrospinal fluid (CSF) dynamics, including the pathophysiology, risk factors, and clinical implications of brain sag after craniotomy.</p> Recent Findings <p>The literature on CSF hypovolemia and brain sag after craniotomy is comprised of single center retrospective studies and case reports with variable definitions. Brain sag, downward displacement of the brain due to CSF hypovolemia and loss of buoyancy, is an under-recognized cause of neurological deterioration after neurosurgery. This occurs most often in the context of perioperative CSF lumbar drainage but has occasionally been reported in the absence of lumbar drainage.</p> Summary <p>CSF hypovolemia and brain sag can cause neurologic deterioration after craniotomy and treatment is opposite that of most other causes of deterioration and brain herniation in this population. For patients with signs of brain sag, immediate interventions such as Trendelenburg positioning and cessation of CSF drainage should be performed. Epidural blood patches or, more rarely, intrathecal saline infusions, have been used in severe cases. More research is needed, particularly on risk factors and effect on longer term outcomes.</p>