In this radiology lecture, we discuss the ultrasound appearance of ovarian dermoid cyst, including the rarely seen but highly specific “floating sphere” sign! Key points include * AKA mature cystic teratoma. * Most common ovarian neoplasm. * Benign, mean age 30. * 10% bilateral. * Mature tissue from ≥2 embryonic germ cell layers: Sebaceous material, hair follicles, skin derivatives, fat, muscle, bone, and other tissues lined by squamous epithelium. * Specificity of US diagnosis 94-100%. * MRI for changing morphology on f/u and for postmenopausal patients. * Ultrasound findings: Floating echogenic spherical structures = “Floating sphere” sign (uncommon but pathognomonic), hyperechoic component with acoustic shadowing (Rokitanksy nodule), hyperechoic lines and dots, fat-fluid levels, diffuse or regional bright echoes. * Most common complication: Ovarian torsion. * Rare complications: Rupture, infection, malignant transformation, hormone secretion, anti-NMDA receptor encephalitis. To learn more about the Samsung RS85 Prestige ultrasound system, please visit: https://www.neurologica.com/rs85-prestige-ultrasound-system-4 Click the YouTube Community tab or follow on social media for bonus teaching material posted throughout the week! Instagram: https://www.instagram.com/radiologistHQ/ Facebook: https://www.facebook.com/radiologistHeadQuarters/ Twitter: https://twitter.com/radiologistHQ