During the month of Ramadan, Muslims fast every day from dawn to sunset. In the healthy subject, this fasting does not have any harmful consequences on health. However, it can induce several complications for patients with diabetes. The aim of this review twofold: first, it seeks to give some clues about methodological aspect of research during Ramadan and to show the impact of various diabetes monitoring and treatment, including biochemical and clinical parameters, diet and caloric intake, drug intake when fasting. Second, it intends to determine whether or not Ramadan fasting induces complications in patients with types 1 and 2 diabetes and ultimately to elaborate some advice as to the management of fasting patients. Several studies have shown that Ramadan fasting did not alter biochemical parameters in patients with type 2 diabetes. However, other studies have shown that there is either an increase or a decrease in biochemical parameters during Ramadan. Ramadan fasting would be acceptable for patients with well-balanced type 2 diabetes who are conscious of their disease and compliant with their diet and drug intake. If patients with type 1 diabetes wish to fast, it is necessary to advise them to undertake control of their glycaemia several times a day. Patients with type 1 diabetes who will fast during Ramadan may be better managed with fast absorption insulin.