Cushing developed many of the basic surgical techniques for operating on the brain, thus establishing himself as one of the foremost leaders and experts of this field. Under his influence neurosurgery became an autonomous surgical discipline. His achievements included:
improving considerably the survival rate of patients after difficult brain operations for intracranial tumors
using x-rays to diagnose brain tumors
using electrical stimuli for study of the human sensory cortex
being the world's leading teacher of neurosurgeons in the first decades of 20th century
Now his name is commonly associated with his most famous discovery - Cushing's disease (hyperadrenocorticism), first described in his work The Pituitary Body and its Disorders (1912). Cushing was also awarded the Pulitzer Prize in 1926 for a biography of one of the fathers of modern medicine - Sir William Osler.
Adapted from: en.wikipedia.org/wiki/Harvey_Cushing