Ultrasound is the primary imaging modality after kidney transplantation. Its general availability and the absence of radiation exposure and discomfort for the patient also make it a suitable procedure for follow-up. Simple B-mode sonography provides comprehensive information on the morphology of the renal transplant and surrounding structures, while color-coded duplex sonography (CCDS) additionally allows assessment of the anastomoses and organ perfusion. More recent technical advances such as power Doppler ultrasound appear to improve in particular the assessment of peripheral circulation. The possibilities and limitations of the various sonographic techniques in the evaluation of kidney transplants are discussed with special reference to the most important clinical aspects and some basic technical concepts.