This book explores representations of girlhood and young womanhood in recent English language children's fantasy by focusing on two fantastic body transformation types: invisibility and age-shifting. Drawing on recent feminist and queer theory, the study discusses the tropes of invisibility and age-shifting as narrative devices representing gendered experiences. The transformations offer various perspectives on a girl's changing body and identity and provide links between real-life and fantastic discourses of gender, power, invisibility and aging. The main focus is on English-language fantasy published since the 1970s but the motifs of invisibility and age-shifting in earlier tales and children's books is reviewed; this is the first study of children's fantasy literature that considers these tropes at length. Novels discussed are from both critically acclaimed authors and the less well known. Most of the novels depicting invisible or age-shifting girls are neither thoroughly conventional nor radically subversive but present a range of styles. In terms of gender, children's fantasy novels can be more complex than they are often interpreted to be.