Michael Herschel Greger (born October 25, 1972) is an American physician, author, and speaker on public health issues best known for his advocacy of a whole-food, plant-based diet, and his opposition to animal-derived food products. He founded the website NutritionFacts.org which provides videos and studies around diet and nutrition. His books How Not to Die, How Not to Diet, and How Not to Age have been New York Times best sellers .

Michael Greger was born on October 25, 1972, in Miami, Florida, United States. Greger has said that he was inspired to pursue a career in medicine at the age of nine after witnessing his grandmother's health improvement that she attributed to following dietary and lifestyle changes prescribed by American nutritionist Nathan Pritikin. He later graduated from the Cornell University School of Agriculture in 1995, where as a junior he wrote informally about the dangers of bovine spongiform encephalopathy on a website he published in 1994. In the same year, he was hired to work on mad cow issues for Farm Sanctuary, near Cornell, and became a vegan after touring a stockyard as part of his work with Farm Sanctuary.

In 1998, Greger appeared as an expert witness testifying about bovine spongiform encephalopathy when cattle producers unsuccessfully sued Oprah Winfrey for libel over statements she had made about the safety of meat in 1996. He later enrolled at Tufts University School of Medicine, originally for its MD/PhD program, but then withdrew from the dual-degree program to pursue only the medical degree. He received his MD in 1999 as a general practitioner specializing in clinical nutrition.

In 2001, Greger joined the Organic Consumers Association to work on mad cow issues, on which he spoke widely as cases of the disease appeared in the US and Canada. Previously in 1994, in a Cornell University animal rights publication, Greger highlighted the results of a survey in Britain that appeared to support the view of a microbiologist at the University of Leeds that mad cow disease was "much more serious than AIDS". A decade later, in early 2004, the Daily Bruin, the student newspaper of the University of California, Los Angeles, reported that Greger had called mad cow disease the "plague of the 21st century". However, Greger later denied ever making such a statement, clarifying that he had merely posed it as a question during a speech. That same year, Greger cited a study and said that "thousands of Americans may already be dying because of Mad Cow disease every year".

In 2004, he launched a website and published a book critical of the Atkins Diet and other low-carbohydrate diets. That same year, the American College of Lifestyle Medicine was founded, and Greger was a founding member, and fellow.

In 2005, Michael Greger joined the farm animal welfare division of the Humane Society of the United States as director of public health and animal agriculture. Three years later, he testified before the United States Congress after the Humane Society released its undercover video of the Westland Meat Packing Company, which revealed downer animals entering the meat supply. This led the USDA to mandate the recall of 143 million pounds of beef, some of which had been routed into the nation's school lunch program.

Greger founded the website NutritionFacts.org, with funding from the Jesse & Julie Rasch Foundation. It was founded in August 2011 to provide information on nutrition and health. Jesse & Julie Rasch Foundation provided the initial seed funding. Greger, then known for public-health lectures and his work with the Humane Society of the United States, aimed to "cut through the hype" by summarizing findings from peer-reviewed studies in an accessible format. From its inception, the site released a new video every weekday, drawing on Greger's Latest in Clinical Nutrition lecture series, to make complex research digestible for the general public.

NutritionFacts.org follows a nonprofit public-service model. It carries no advertisements and sells no products; operating costs are covered by individual donations and philanthropic grants. Its core service is its video library: referenced videos, most narrated by Greger, which summarizes recent nutrition studies. Each video is accompanied by a transcript and citation list, which helps readers to trace the original research.