Previously, studies of the endocrine pancreatic β-cell were mainly performed ex vivo by morphological means. This data supported the analyis of pathophysiological changes in the pancreatic islet during insults such as diabetes mellitus. Metabolic testing of the pancreatic islet by assaying hormone parameters such as plasma insulin or C-peptide combined with more or less sophisticated calculations allowed conclusions about states of insulin resistance or secretory failure. It also allowed certain correlations of endocrine function with β-cell mass. Today, with firmer pathophysiological concepts about β-cell failure, modern protocols of islet transplantation, and drugs on the market coming with promises of preservation or even expansion of β-cell mass in diabetes mellitus it has become very attractive to search for tools measuring β-cell mass, if possible even repeatedly in the same organism in vivo. From a clinical point of view, the potential of pancreatic β-cell mass imaging technologies is looked upon with high expectations. Methodologically, the decisive question is whether it is likely that future β-cell imaging will provide significant advantages over the metabolic methods already in hand. With new in vivo tools, studies of β-cell mass and function may offer even new approaches stratifying patients to anti-diabetic therapies.