How oestrogen levels rise and fall across a lifetime, from before puberty to after menopause. The chapter explains why levels stay low in childhood while the hypothalamus is still developing, then how a surge at puberty triggers changes such as breast development and the start of periods. It follows oestrogen through the menstrual cycle, describing the signals between the hypothalamus, pituitary gland and ovaries, the peak before ovulation and how oestrogen works with progesterone afterwards. It also covers pregnancy, when oestrogen is higher than at any other time, and the decline at menopause that causes most menopausal symptoms and why hormone replacement may be used.