The islets of Langerhans are key regulators of glucose homeostasis and have been known as a structure for almost one and a half centuries. During the twentieth century several different cell types were described in the islets of different species and at different developmental stages. Six cell types with identified hormonal product have been described so far by the use of histochemical staining methods, transmission electron microscopy, and immunohistochemistry. Thus, glucagon-producing α-cells, insulin-producing β-cells, somatostatin-producing δ-cells, pancreatic polypeptide-producing PP-cells, serotonin-producing enterochromaffin-cells, and gastrin-producing G-cells have all been found in the mammalian pancreas at least at some developmental stage. Species differences are at hand and age-related differences are also to be considered. Eleven years ago a novel cell type, the ghrelin cell, was discovered in the human islets. Subsequent studies have shown the presence of islet ghrelin cells in several animals, including mouse, rat, gerbils, and fish. The developmental regulation of ghrelin cells in the islets of mice has gained a lot of interest and several studies have added important pieces to the puzzle of molecular mechanisms and the genetic regulation that lead to differentiation into mature ghrelin cells. A body of evidence has shown that ghrelin is an insulinostatic hormone, and the potential for blockade of ghrelin signalling as a therapeutic avenue for type 2 diabetes is intriguing. Furthermore, ghrelin-expressing pancreatic tumours have been reported and ghrelin needs to be taken into account when diagnosing pancreatic tumours. In this review article, we summarise the knowledge about islet ghrelin cells obtained so far.