Diabetic nephropathy is a major complication of diabetes and remains a common health problem worldwide. It is the dominant cause of incident end-stage renal disease. Currently, microalbuminuria is an only noninvasive marker available for the diagnosis of diabetic nephropathy. However, some patients with microalbuminuria have advanced renal pathological changes for which therapy is less effective than earlier stages of the disease. Additionally, the immunoassay to measure microalbuminuria can detect only immunoreactive forms of albumin, whereas immunounreactive forms are undetectable by this conventional method. Therefore, novel biomarkers for earlier diagnosis of diabetic nephropathy are crucially required. Moreover, the incidence rate of diabetes-induced end-stage renal disease remains high despite good control of blood sugar levels and adequate treatment with appropriate regimens, indicating the need of new therapeutic targets for better therapeutic outcome and successful prevention of diabetic nephropathy in diabetes patients. During the proteomic era, proteomics has become a powerful tool for unraveling the disease pathophysiology and for biomarker discovery. This chapter summarizes recent studies that applied proteomics for the investigation of diabetic nephropathy with major aims to search for novel biomarkers and new therapeutic targets.