The popularity of the motion picture soundtrack O Brother, Where Art Thou? brought an extraordinary amount of attention to bluegrass, but it also drew its share of criticism from some aficionados who felt the album’s inclusion of more modern tracks misrepresented the genre. This soundtrack, these purists argued, wasn’t bluegrass, but “roots music,” a new and, indeed, more overarching category concocted by journalists and marketers. Why is it that popular music genres like these and others are so passionately contested? And how is it that these genres emerge, coalesce, change, and die out?