The advent of ultrasound and computed tomography resulted in a great increase in detection and diagnosis of small renal parenchymal tumors. These are mainly slow growing tumors, without metastatic disease and with possible multicentricity at the diagnosis. Moreover there is not agreement about the best treatment for the small (< 3 cm) renal cell carcinoma. In this paper the role of ultrasound, computed tomography and magnetic resonance for detection and characterization of the small renal masses is discussed. On occasion it is possible to obtain the tissue characterization of a solid renal mass by diagnostic imaging (for example angiomyolipomas); however, most frequently, solid renal masses have an aspecific appearance. The majority of problematic renal masses have cystic components. The diagnosis of simple cyst is based on few simple but rigid criteria: homogeneous water density, very thin wall, well defined and sharp interface with renal parenchyma, lack of contrast enhancement. When there are intracystic septae, thickened wall or increased density, the cyst is "complicated". In these cases the classification suggested by Bosniack can be helpful. Bosniack class-1-lesions are simple cysts; they do not require any further work-up. Bosniack class-2-lesions are minimally complicated but reliably benign cysts (thin wall, thin calcifications, thin septae). Some of these lesions require follow-up; and the majority of them do not. Class-3-lesions have thick septae, thick calcifications and thick and irregular walls, but not contrast enhancement. In most cases these lesions require surgical exploration for diagnosis and therapy. Bosniack class-4 lesions are clearly malignant; they are indicated by contrast enhancing regions within cysts. They always require surgery.