β cell dysfunction is sufficient to cause hyperglycemia; β cell loss is not necessary but, if severe, can be sufficient and may be accompanied by intrinsic β cell dysfunction. Clinical testing can differentiate β cell capacity from β cell glucose sensitivity but cannot ascribe either to relative changes in β cell mass versus function. However, longitudinal and intervention studies indicate that β cell glucose insensitivity (stunning) closely tracks with hyperglycemia and is, at least in part, reversible. Rescuing stunned β cells is a key therapeutic target.