Very low energy diets (VLEDs) are defined as diets which contain energy levels of less than 3.4 MJ (800 kcal) per day and contain daily allowances of all essential nutritional requirements. These diets have been in clinical use for more than 20 years. They are used as the only source of nutrition for 8-16 weeks, which usually achieves a weight loss of 1.5-2.5 kg per week. Before using this type of diet a medical investigation is necessary to evaluate contraindications and to check medication use during the diet. To facilitate maintenance, cognitive behavioural counselling should always be included in a weight reduction programme using a very low energy diet. VLEDs have no serious harmful effects and can safely be used in patients with various chronic diseases. Programmes using VLEDs produce better short-term weight loss than programmes without the diet. However, in randomized controlled trials VLED-based programmes have not achieved significantly better long-term maintenance than conventional programmes. VLEDs are used when rapid weight loss is necessary because of an obesity-related disease. In other patients with obesity it is an alternative to other conservative approaches for treatment of obesity. In type 2 diabetes it may improve long-term glucose metabolism better than conventional weight reducing diets. Some studies suggest that after a VLED-based programme long-term maintenance is better among men than women. This possible gender difference is an important topic for further research.