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Craniotomy is a cut that opens the cranium. During this surgical procedure, a section of the skull, called a bone flap, is removed to access the brain underneath. The bone flap is usually replaced after the procedure with tiny plates and screws.

A craniotomy may be small or large depending on the problem. It may be performed during surgery for various neurological diseases, injuries, or conditions such as brain tumors, hematomas (blood clots), aneurysms or AVMs, and skull fractures. Other reasons for a craniotomy may include foreign objects (bullets), swelling of the brain, or infection. Depending on the reason for the craniotomy, this surgery requires a hospital stay that ranges from a few days to a few weeks.

In the Medicine, a Neurosurgeon's hands are referred to as "the hands of God". 

In this picture, a large benign cranial tumor is being excised.