Colour coded duplex sonography (CCDS) is the method of choice in diagnostic monitoring of kidney transplants. Lack of radiation exposure, easy accessibility and high resolution have contributed to the fact that CCDS is used to monitor kidneys after transplantation. However, the value of CCDS in diagnosis of rejection is still a matter of debate. CCDS is used to evaluate perfusion of transplanted kidneys, detect renal artery stenosis or real vein thrombosis and arteriovenous fistulas. The value of recent technical advances, such as 3D Sonography, contrast harmonic imaging and tissue harmonic imaging, is currently being tested. This article focuses on indications and limitations of CCDS in the diagnosis of functional impairment of transplanted kidneys.