Follicle monitoring is a method of fertility assistance to improve conception rates by monitoring follicle growth, endometrial morphology and ovulation by ultrasound during the menstrual cycle to guide couples on the timing of intercourse. Follicle monitoring is non-invasive to the human body, fits the natural state of conception, has low economic cost and is accepted by the majority of couples with fertility needs. For patients with menstrual disorders, anovulatory menstruation, polycystic ovary syndrome and other non-tubal factors causing infertility, follicle monitoring is generally the first step in scientific pregnancy assistance if the male partner’s semen is generally normal. Here we briefly introduce follicle monitoring: The dominant follicle is the one selected by multiple factors in the natural cycle, which is the fastest growing, largest in size and able to mature and expel under appropriate conditions. The growth rate of the dominant follicle is 1~2mm/day. The follicles are observed to be 17-23mm in diameter under ultrasound and have a full, round or oval shape with high tension, thin capsule walls and good translucency, which is a sign of follicular maturation. For women with regular menstruation (28-35 days cycle), monitoring is usually started from 8-10 days of menstrual cycle and the time of next monitoring is decided according to the size of follicles, usually 2-3 days. When the diameter of the dominant follicle reaches 16mm or more, monitoring is required once every 1-2 days. For women with irregular periods, follicle monitoring usually starts on the second or third day of menstruation, after which the timing of monitoring is determined by the growth of the follicles and the medication used. Follicle monitoring is often supplemented with urinary LH (luteinizing hormone) monitoring. The LH peak in the blood is the command to induce ovulation. We use LH test strips to monitor the LH peak in the urine and ovulation occurs within 48 hours of the peak. For patients with ovulation disorders, ovulation can be induced by intramuscular injection of human chorionic gonadotropin (hCG), which also allows for better control of ovulation time. 48 hours later, an ultrasound is performed to see if the follicle has been ovulated. The best time to instruct intercourse is between 2 days before and one day after ovulation. Couples who have not conceived after 3-4 cycles may consider further treatment such as IUI or IVF. As each patient’s underlying condition is different, some patients have poor endometrial morphology, some have combined polycystic ovary syndrome or follicular maturation disorder, etc. These patients are often accompanied by some pharmacological interventions such as estradiol, letrozole, hMG, etc. during follicular monitoring. The process of ovulation is complex and full of variables. Follicle monitoring under ultrasound can visualize the growth status of follicles and the endometrial status, which hopefully can bring the most help to couples with fertility needs.