A renal cyst is a round, fluid-filled sac that develops in or on the kidney. Simple renal cysts are extremely common, especially as people age, and are almost always benign. On ultrasound, a simple renal cyst has characteristic features: it is completely anechoic (black), has a thin, smooth wall, shows posterior acoustic enhancement, and contains no internal echoes, septations, or solid components. When all these criteria are met, the cyst is classified as Bosniak category I and requires no further workup. Complex cysts — those with septations, calcifications, thick walls, or solid components — are classified into higher Bosniak categories and may require additional imaging or even biopsy to rule out malignancy. Ultrasound is the primary tool for detecting and characterizing renal cysts.