Cystic renal lesions can be classified as either simple or complicated cysts, which might occur as solitary as well as multifocal lesions. The Bosniak classification (I-IV), which characterizes renal cysts on the basis of ultrasound or computer tomographic criteria, is very useful for further decision-making about the therapeutic approach. The method of choice for diagnosis of renal cysts is ultrasound. Besides the conventional B-mode ultrasound, contrast enhanced ultrasound with SonoVue provides a promising new technique for distinguishing cysts according to the Bosniak classification. This review describes cystic renal lesions with emphasis on the etiology and significance of these pathologies in a methodological comparison of conventional B-mode, contrast enhanced ultrasound and computer tomography.