Fasting, intermittent or continuous, religious or therapeutic, is knowing a growing craze. Despite few randomized controlled studies, therapeutic fasting is prescribed in various chronic diseases, as diabetes, hypertension and also cancer. Fasting is applied to lose weight in overweight and obese patients. However, weight loss is often associated with fat-free mass loss. Chronic caloric restriction has been associated with longevity in animal studies, while it has been poorly studied in humans to date. Good quality studies are needed to better understand the effects of fasting on health and diseases.