Urinary excretion of human serum albumin (HSA), a 6.65 kDa monomeric protein, is a sensitive marker of renal damage associated with many diseases including diabetes mellitus. Albumin is synthesized by the liver and functions as a transport protein for fat-soluble hormones and drugs and for maintaining plasma colloid osmotic pressure and pH. Albumin is not filtered at the glomerulus and its presence in the urine at concentration above 30 mg/day is suggestive of glomerular damage. Early diagnosis of microalbuminuria (30–300 mg/24 h urine albumin excretion or 30–300 mg/g creatinine in random collections) has prognostic value for monitoring disease progression and early clinical management of diabetic nephropathy in prediabetic patients. Current methods for quantitation of urine albumin are based on immunoassays or size exclusion high-performance liquid chromatography coupled with UV detection (SEC-HPLC-UV). Studies have demonstrated discordance between the existing methods. It has been suggested that while immunoassays underestimate albumin in urine, SEC-HPLC-UV method overestimates albumin as it cannot separate co-eluting interferences. This chapter describes a liquid chromatography tandem mass spectrometry LC-MS/MS candidate reference method for albumin quantitation.