Sonography is a widely used modality for the evaluation of both native and transplanted kidneys. It is noninvasive, portable, and requires minimal patient preparation. Renal sonography can estimate kidney size, determine the presence or absence of hydronephrosis, and the presence and characteristics of any intrarenal or extrarenal masses. The addition of color, and, more recently, power Doppler have enhanced the diagnostic capabilities of renal sonography. Color and power Doppler have distinct but complementary uses, and knowledge of the advantages and limitations of each are essential for proper application of these powerful tools. The differences between color and power Doppler is discussed, with emphasis on their relative strengths. Clinical uses of color Doppler include the evaluation of perfusion abnormalities, renal artery stenosis, renal vein thrombosis, pseudoaneurysms, and arteriovenous fistulas. Color and power Doppler are also helpful in the evaluation of the transplanted kidney and can suggest the presence of transplant rejection. The sonographic color and power Doppler features of disease entities which affect the kidneys are discussed. Knowledge of these sonographic features will enable prompt diagnosis, thereby expediting patient care.