The most common renal mass is a cyst, or fluid-filled sac arising from the kidney. Cysts are common and do not typically cause problems or carry any significant health risks. Most cysts are easily identified by the radiologists and can be classified as simple cysts, which don’t need intervention, and may not even need further follow-up or observation. Solid masses are worrisome for cancer; radiologists easily identify these. These masses are cancerous about 80% of the time. About one in five masses with characteristics suspicious for cancer on the imaging test will not be cancerous, but is generally treated with the expectation that it is cancerous. Occasionally a CT- or ultrasound-guided biopsy of the mass is recommended to evaluate the mass more thoroughly. Sometimes a biopsy is helpful in guiding further therapy. However, the biopsy is frequently not as helpful as one might think. If the biopsy shows cancer, as predicted by the scan images, the subsequent management has not been altered and surgery or ablation is required.