The term 'new adult' was coined in 2009 by St Martin's Press, when they sought submissions for a contest for 'fiction similar to YA that can be published and marketed as adult - a sort of 'older YA' or 'new adult'.' However, the literary category that later emerged bore less resemblance to young adult fiction and instead became a sub-genre of another major popular genre: romance. This Element uses new adult fiction as a case study to explore how genres develop in the twenty-first-century literary marketplace. It traces new adult's evolution through three key stages in order to demonstrate the fluidity that characterises contemporary genres. It argues for greater consideration of paratextual factors in studies of genre. Using a genre worlds approach, it contends that in order to productively examine genre, we must consider industrial and social factors as well as texts.