This chapter tries to address the fascinating interplay between the glucose-insulin system and the growth hormone (GH)-insulin-like growth factor 1 system from the angle of GH excess. As these two systems are so closely linked, one might expect that when GH secretion is pathologically increased, the glucose-insulin system must adapt to this - and indeed it does. In acromegaly, diabetes - or at least impaired glucose tolerance - is the result of GH excess. The medical treatment modalities for acromegaly have interesting and diverse effects on glycemic control. As many of these aspects are also important for the discernment of the metabolic differences between type 1 and type 2 diabetes mellitus, we believe that the reader might obtain a better understanding of the (patho)physiology of the link between glycemic control and GH secretion.