Ovarian follicles appear on ultrasound as small anechoic cysts within the ovarian stroma. A dominant follicle grows to 18–28 mm before ovulation and is tracked during fertility monitoring. The mature (Graafian) follicle collapses at ovulation, and a corpus luteum with internal echoes and peripheral vascularity forms in its place. Multiple follicles of varying sizes are seen in polycystic ovary syndrome (PCOS). Follicle monitoring is a routine component of assisted reproduction protocols.