A healthy diet is a diet that helps to maintain or improve overall health. A healthy diet provides the body with essential nutrition: fluid, macronutrients, micronutrients, and adequate calories. The requirements for a healthy diet can be met from a variety of plant-based and animal-based foods, although a non-animal source of vitamin B12 is needed for those following a vegan diet. A healthy diet supports energy needs and provides for human nutrition without exposure to toxicity or excessive weight gain from consuming more calories than the body requires. A healthy diet, in addition to exercise, may lower disease risks, such as obesity, heart disease, type 2 diabetes, hypertension and cancer.

Various nutrition guides are published by medical and governmental institutions to educate individuals on what they should be eating to promote health. Nutrition facts labels are also mandatory in some countries to allow consumers to choose between foods based on the components relevant to health.

The idea of dietary therapy (using dietary choices to maintain health and improve poor health) is quite old and thus has both modern scientific forms (medical nutrition therapy) and prescientific forms (such as dietary therapy in traditional Chinese medicine).