While the MG category has its own gray areas, the YA category is especially tricky to define. It was only a decade ago, following the publication of series such as Twilight and Hunger Games, that YA became a publishing phenomenon and was given a stand-alone bestseller list by the New York Times. It was just two decades ago that the Harry Potter books, widely considered middle grade, began dominating the book market. What makes a book middle grade or young adult today? The answer depends on several factors, as agents and publishers explain.