Bob Dylan's influence on popular music is incalculable. As a songwriter, he pioneered several different schools of pop songwriting, from confessional singer/songwriter to winding, hallucinatory, stream-of-conscious narratives. As a vocalist, he broke down the notions that in order to perform, a singer had to have a conventionally good voice, thereby redefining the role of vocalist in popular music. As a musician, he sparked several genres of pop music, including electrified folk-rock and country-rock. And that just touches on the tip of his achievements. Dylan's force was evident during his height of popularity in the '60s -- the Beatles' shift toward introspective songwriting in the mid-'60s never would have happened without him -- but his influence echoed throughout several subsequent generations. Many of his songs became popular standards, and his best albums were undisputed classics of the rock & roll canon. Dylan's influence throughout folk music was equally powerful, and he marks a pivotal turning point in its 20th century evolution, signifying when the genre moved away from traditional songs and toward personal songwriting. Even when his sales declined in the '80s and '90s, Dylan's presence was calculable.