As we learn more about the pathophysiology of diabetes mellitus, we find that there is more yet to be learned. This may sound like a trite statement, but in reality it is true. The following article reviews the basic pathophysiology of both type 1 diabetes mellitus and type 2 diabetes mellitus as we understand it today. It continues on to reveal the “things that go wrong” when there is too much or too little glucose available to the body organs and especially to the brain. The article points out the signs and symptoms to be aware of when the person is in the acute state of diabetic ketoacidosis, hyperglycemic hyperosmolar nonketotic coma (or state), and severe hypoglycemia. It concludes with important considerations when the individual is in one of these acute states and contributes key points related to the control of diabetes when the person is in the state of compromise.