Most renal masses are benign cysts; a subset are malignant. Most renal masses are incidental findings. Evaluation of renal cysts has evolved with updates to the Bosniak classification system and other guidelines. The Bosniak classification provides detailed definitions and extends the system from computed tomography to MR imaging. This article provides a simple approach to the evaluation of cystic or potentially cystic renal masses. The radiologist is central to this process. Key elements include confirming that a renal lesion is cystic and not solid, determining the need for further characterization by imaging, and judicious application of the Bosniak classification system.