Diabetes mellitus is a heterogeneous disease that might present as insulin-dependent (type I), non-insulin-dependent (type II), and other forms associated with other endocrine and genetic conditions, as well as those secondary to different infiltrative or destructive pancreatic diseases, parasitosis and post-pancreatectomy patients. Insulin-dependent diabetes mellitus is mainly associated to immunological problems, probably by derangement caused by external factors, such as viral infections that favor destruction of beta cells of the pancreas. On type II diabetes the hereditary component is more important and does not present immunological problems. Different etiological considerations are included, as well as associated genetic markers that are described in the text.