Kidney disease, characterized by proteinuria and glomerular lesions, is a common complication of spontaneous diabetes mellitus in many animal species. It occurs in animals with hypoinsulinemia, hyperinsulinemia, or impaired glucose tolerance. The renal functional and structural abnormalities in spontaneously diabetic animals resemble human diabetic nephropathy in many respects. Mesangial expansion and glomerular basement membrane thickening, two structural hallmarks of diabetic glomerulopathy in humans, are the most frequently encountered lesions in animals. In addition, a nodular form of mesangial expansion that resembles but is not identical with human nodular glomerulosclerosis or the Kimmelstiel-Wilson lesion has been observed in some animal models. Other abnormalities, such as exudative hyaline lesions and arteriolar hyalinosis, have also been noted occasionally in other models. Although diabetic animals may develop kidney disease that resembles human diabetic nephropathy, no single animal model develops renal changes identical to those seen in humans. Nonetheless, animal models with spontaneous diabetic kidney disease may be useful for investigating the mechanisms of development of diabetic nephropathy and the effects of various treatment modalities on the progression of renal disease.