Walter Willett is an American epidemiologist and nutritionist known for his research on diet, lifestyle, and health. He is a professor at Harvard T.H. Chan School of Public Health and Harvard Medical School, where he has contributed significantly to understanding the relationship between nutrition and chronic diseases. Willett is also a key figure in the Nurses' Health Study, one of the largest and longest-running studies of women's health. His work has influenced dietary guidelines and public health policies, emphasizing the importance of a balanced diet and healthy lifestyle choices for disease prevention.