My dissertation examines the alternative worlds in recent fantasy fiction for young adults in Britain and France. I compare J. K. Rowling’s Harry Potter series (1997-2007) and Philip Pullman’s His Dark Materials (1995-2000) trilogy to two French fantasy series, Erik L’Homme’s Le Livre des Étoiles ( The Book of the Stars) (2001-2003) and Pierre Bottero’s Ewilan (2003-2007) series. My work contributes to scholarship on French children’s literature and reflects some of the ways in which comparative approaches to writing for children and young adults can highlight the differences in literary traditions as new genres, such as the rise of la fantasy in France, emerge as acceptable modes of literary production. Via the spatiality theories of Michel Foucault, Michel de Certeau and others, I argue that the alternative worlds in these texts constitute training grounds that prepare protagonists and their readers to develop codes of ethical action. Focusing on fantasy allows me to explore and compare two very different traditions of writing for children and adolescents. While the British “Golden Age” of children’s fiction led to works such as Alice in Wonderland, Peter Pan, Winnie-the-Pooh and later to C.S. Lewis’ Chronicles of Narnia, a similar shift towards imaginary, fantasy worlds did not occur in France, where writing for children and young adults was grounded in instructional strategy. I argue that in current fantasy narratives, protagonists model resistant and subversive strategies that readers can also use to oppose abuses of power in their own world. What is at stake in these texts is not the traditional binary between light and dark or good and evil. Through the mental and moral growth of their protagonist-heroes, fantasy authors are teaching their readers of all ages to think critically about responding to moral injustices and ethical uncertainties in their own world and in their everyday lives. By imagining these impossible worlds, fantasy provides readers of all ages with the mental practice necessary for becoming capable and mature problem-solvers, who are equipped to face the challenges and problems in their own world with bravery and hope.