Estrogen is a category of sex hormone primarily responsible for the development and regulation of the female reproductive system and secondary sexual characteristics. The main types of estrogen are estrone (E1), estradiol (E2), estriol (E3), and estetrol (E4). Estradiol is the most potent and prevalent form during the reproductive years, estriol is predominant during pregnancy, and estrone is more common after menopause. Estrogen is synthesized in various tissues, including the ovaries, adrenal glands, and fat tissues. It is present in both females and males, though at significantly higher levels in females of reproductive age. Biologically, estrogen regulates the menstrual cycle, promotes the development of female secondary sexual characteristics such as breasts and changes in body composition, maintains vaginal wall thickness, and prepares the endometrium for potential pregnancy. It also influences bone density, cholesterol levels, glucose metabolism, skin, hair health, and mood. In males, estrogen contributes to reproductive functions, including sperm maturation and libido. At the cellular level, estrogen diffuses into cells, binds to estrogen receptors, and modifies gene expression, which leads to its diverse physiological effects. Estrogen is also used medically in treatments such as menopausal hormone therapy, hormonal birth control, and feminizing hormone therapy. Both high and low levels of estrogen can lead to health issues. High levels may cause acne, depression, and an increased risk of certain cancers, while low levels may lead to weight gain, cardiovascular disease, and symptoms like hot flashes and mood changes. In summary, estrogen is a vital hormone involved in reproductive health and other physiological processes in both women and men. Its balance is crucial for overall well-being.