Although proteinuria is usually benign in the form of transient or orthostatic proteinuria, persistent proteinuria may be associated with more serious renal diseases. Proteinuria may be an independent risk factor for the progression of chronic kidney disease in children. Mechanisms of proteinuria can be categorized as glomerular, tubular, secretory, or overflow. A history, a physical examination, and laboratory tests help determine the cause. Transient (functional) proteinuria is temporary. It can occur with fever, exercise, stress, or cold exposure, and it resolves when the inciting factor is removed. Orthostatic proteinuria is the most common type in children, especially in adolescent males. It is a benign condition without clinical significance. Persistent proteinuria can be glomerular or tubulointerstitial in origin. The urine dipstick test is the most widely used screening method. Although a 24-hour urine protein excretion test is usually recommended for quantitation of the amount of protein excreted in the urine, it may be impractical in children. A spot, first-morning urine test for a protein-to-creatinine or protein-to-osmolality ratio is a reliable substitute. Treatment of proteinuria should be directed at the underlying cause. Patients with active urinary sediments, hematuria, hypertension, hypocomplementemia, renal insufficiency with depressed glomerular filtration rate, or signs and symptoms suggestive of vasculitic disease may require referral to a pediatric nephrologist and a renal biopsy.