The causes of an inadequate control of glucose metabolism in diabetes patients can vary greatly. In order to identify the underlying cause, a detailed medical history, a thorough physical examination and laboratory diagnostics focussed on the suspected diagnosis are required. The therapeutic measures initiated should be aimed at both adjusting the glucose-lowering therapy and treating the underlying disease. The aim of this review is to provide an overview of possible causes of inadequate metabolic control in diabetes patients and the necessary diagnostic and therapeutic measures.