Muesli was introduced around 1900 by the Swiss physician Maximilian Bircher-Benner for patients in his hospital, where a diet rich in fresh fruit and vegetables was an essential part of therapy. It was inspired by a similar "strange dish" that he and his wife had been served on a hike in the Swiss Alps. Bircher-Benner himself referred to the dish simply as "d'Spys" (Swiss German for "the dish", in German "die Speise"). Muesli in its modern form became popular in western countries starting in the 1960s as part of increased interest in health food and vegetarian diets. Traditional muesli was eaten with orange juice and not milk.

Packaged muesli is a loose mixture of mainly rolled oats and/or cornflakes together with various pieces of dried fruit, nuts, and seeds. It commonly contains other rolled cereal grains such as wheat or rye flakes as well. There are many varieties, some of which may also contain honey, spices, or chocolate. This dry packaged muesli can be stored for many months. It can be served quickly after mixing it with milk, fil, yogurt, coffee, hot chocolate, fruit juice, or even just plain water. If desired, pieces of fresh fruit may be added. Alternatively, the mix may be soaked overnight in milk and then served with fresh fruit or compote to taste.