A simple cyst is a benign fluid-filled structure that meets specific sonographic criteria: it is anechoic (no internal echoes), has a thin imperceptible wall, demonstrates posterior acoustic enhancement, and shows no internal septations, nodules, or vascularity on Doppler. Simple cysts are common incidental findings in the liver, kidneys, and ovaries. When all sonographic criteria are met, no further workup is required. Cysts that deviate from these criteria — with debris, septations, or wall irregularity — require further characterization.