Power. Gender. Sustainability. This Element harnesses powerful new data about gender and sustainability, presents inspiring stories of empowerment, and introduces a framework for building empowerment muscles. First, from a pioneering global survey, it unveils three shocking truths about young women's empowerment. It also compiles significant data on systemic gender disempowerment intersecting environmental degradation, violence, and exclusion, as well as profound societal impact if girls and women were fully empowered. Second, from climate activist Greta Thunberg to the all girl Afghan robotics team, the #NeverAgain movement against gun violence, and the Hong Kong pro-democracy movement, today's empowered girls are a transformative force for change. Each modeling a distinct skill - an empowerment muscle - seven case studies present empowerment muscles of focus, solidarity, hope, courage, advocacy, endurance, and healing. Third, unlike most works using empowerment nebulously, this Element concretizes empowerment - a set of muscles each reader can build and strengthen through 'workout' training exercises.