Estrogens are a group of steroid compounds, named for their importance in the oestrus cycle, and functioning as the primary female sex hormone. Estrogens are used as part of some oral contraceptives, in oestrogen replacement therapy of postmenopausal women, and in hormone replacement therapy for transwomen. Like all steroid hormones, estrogens readily diffuse across the cell membrane; inside the cell, they interact with oestrogen receptors. Additionally, estrogens have been shown to activate a G protein-coupled receptor. This book gathers leading research from around the world.