Amenorrhoea is the absence of menstrual periods in a woman of reproductive age. There are two main types: primary amenorrhoea, where periods do not start by age 16, and secondary amenorrhoea, where established periods stop for six months or more. Common causes of secondary amenorrhoea include pregnancy, breastfeeding, contraceptive use, stress, weight change, excessive exercise, and medical conditions affecting hormone levels like polycystic ovary syndrome. Treatment depends on the underlying cause but may involve lifestyle changes, hormone therapy, weight management, or fertility treatment.