Hardcore hip hop, also referred to as hardcore rap, is a sub-genre of hip hop music that developed through the East Coast hip hop scene in the 1980s. Pioneered by such artists as Schoolly D, Boogie Down Productions, and Kool G Rap, it is generally characterized by an aggressive sound and confrontational lyricism, while the form has often been associated with gangsta rap. However, hardcore hip hop does not generally revolve around "gangsta" lyrical themes, even though there is a great deal of overlap, especially among hardcore rappers of the 1990s. While hardcore rap has been associated with a "monolithic 'gangsta' outlook" by the popular press, hardcore rappers have laid claim to a wide variety of ground. Following a breakthrough in the early 1990s, hardcore hip hop became a popular crossover style during the mid-1990s.