More than two centuries ago, Alexander Monro applied some of the principles of physics to the intracranial contents and for the first time hypothesized that the blood circulating in the cranium was of constant volume at all times. This hypothesis was supported by experiments by Kellie. In its original form, the hypothesis had shortcomings that prompted modification by others. What finally came to be known as the Monro-Kellie doctrine, or hypothesis, is that the sum of volumes of brain, CSF, and intracranial blood is constant. An increase in one should cause a decrease in one or both of the remaining two. This hypothesis has substantial theoretical implications in increased intracranial pressure and in decreased CSF volume. Many of the MRI abnormalities seen in intracranial hypotension or CSF volume depletion can be explained by the Monro-Kellie hypothesis. These abnormalities include meningeal enhancement, subdural fluid collections, engorgement of cerebral venous sinuses, prominence of the spinal epidural venous plexus, and enlargement of the pituitary gland.