As renal cell carcinoma (RCC) is the most common solid renal neoplasm, diagnosis of a renal mass is virtually a differentiation of RCC from other tumors. A solid mass in the kidneys may be considered RCC unless strong evidence suggests another diagnosis. Cystic RCCs comprise a smaller portion of all cystic renal masses, but differentiation of cystic RCC from benign renal cysts is critical to management. The majority of RCCs are incidentally detected by imaging studies, such as ultrasonography (US) or computed tomography (CT). CT is the dominant imaging modality for staging and surgical planning. CT can also provide information for the differentiation of RCC subtypes, which is important in planning treatment and closely related to prognosis. Magnetic resonance imaging (MRI) is used as a problem-solving modality and for staging. Lymphomas, metastases, and various sarcomas are other solid tumors of the kidney. As lymphomas and metastases have unique clinical settings, most cases can be diagnosed easily. Sarcomas commonly manifest as large masses in young patients.