US plays an extremely important role in the diagnosis of renal masses. However, a number of diagnostic difficulties still exists in both identification of the mass and definition of its nature. The most frequent causes of diagnostic errors are discussed. Errors may be due to the radiologist, to technical limitations, and to the lesion itself. The radiologist is responsible for inexperience and negligence (incorrect application of the technique, limited clinical information, poor knowledge of US findings). Technical limitations are due to poor spatial and contrast resolution, to extremely fat patients, and to artifacts. As far as lesions are concerned, cystic and solid masses must be distinguished, since the problems are different. As for cystic lesions, problems are relative to their visualization and to the definition of their nature in cases of atypical or complex cysts, due to the complexity of some US findings. The latter involve both cystic wall and content and are related to calcifications, septa, vegetations, blood, purulent debris. In case of solid masses, problems concern the identification of small renal tumors, the differentiation among the various anatomical variants, the differential diagnosis of benign from malignant tumors, and the evaluation of tumor extent. The authors conclude that, whereas operator-dependent errors can be avoided, those inherent to technical parameters and to the lesion itself represent the diagnostic limitation of US.