 He is also a professor of medicine at Harvard Medical School.Willett is the principal investigator of the second Nurses' Health Study (NHS2 or NHS II), a compilation of studies regarding the health of older women and their risk factors for major chronic diseases.He has published more than 1,500 scientific articles regarding various aspects of diet and disease and is the second most cited author in clinical medicine..

Willett is perhaps best known for his 2001 book "Eat, Drink, and Be Healthy", which presents nutritional information and recommendations based on what was then the consensus of nutrition scientists.