Dr. Mache Seibel’s book, The Estrogen Window, introduces a new era of understanding hormone replacement therapy as well as a new level of clarity for controversial topics of medicine. The forces that coax experts towards trends and habits and pull them away from hard science are insurmountable to nearly all. Dr. Mache manages to give us an unbiased read on the clear message from the vast world of hormone replacement research. He gives a case that will satisfy the most technical readers and summarizes his perspectives into action steps that give clear direction to both experts and lay audiences. This is now the top book on hormone replacement that I will recommend to my patients and peers.