Whoever said progress was a slow process wasn't talking about hip-hop. The art form that grew from block parties in the Bronx to a monumental force in popular music in a matter of years did so mainly by virtue of constant reinvention. And while innovation never moved forward in one simple straight line, there have always been certain songs that made a singular, undeniable impact. 

Sometimes the songs happened to also be one of the best songs by the given artist, as is the case with Biggie and "Juicy." Other times they weren't even the artist's signature song, as is the case with Biz Markie. And some of the songs were by made by artists who would not go on to have legendary careers, as is the case with Soulja Boy. But in every case, these songs led to a significant, measurable change in the genre. And although there are many songs that have shaped hip-hop, we chose to focus on a select few. Here, in chronological order of release, are 20 Songs That Changed Rap Forever.

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