Type 1 and type 2 diabetes are caused by the destruction of pancreatic islet β cells or the reduction of their function and mass (β-cell mass, BCM). Therefore, early detection of BCM is vital for the diagnosis and treatment of diabetes. The molecular imaging methods are an important way to detect BCM, mainly including optical imaging, magnetic resonance imaging (MRI), radionuclide imaging, etc. In recent years, specific imaging of islet β cells by utilizing the radionuclide-labelled glucagon-like peptide-1 (GLP-1) and its analogues has achieved remarkable achievements. This paper reviews novel molecular imaging methods for the detection of islet BCM.