After more than a decade of stability or improvement, the mental health of adolescents plunged in the early 2010s, with rates of depression, anxiety, self-harm, and suicide rising sharply. The author lays out the facts about the epidemic of teen mental illness that hit many countries at the same time, and then investigates the nature of childhood, including why children need play and independent exploration to mature into competent, thriving adults. He presents more than a dozen mechanisms by which this rewiring of childhood has interfered with children's social and neurological development, covering everything from sleep deprivation to attention fragmentation, addiction, loneliness, social contagion, social comparison, and perfectionism. He explains why social media damages girls more than boys and why boys have been withdrawing from the real world into the virtual world, with disastrous consequences. He describes steps that parents, teachers, schools, tech companies, and governments can take to end the epidemic of mental illness and restore a more humane childhood.