Bluegrass began in late-'40s Appalachia when Kentucky singer/mandolinist Bill Monroe and his Bluegrass Boys inflected country tunes with mountain music, based in both Celtic folk and Southern blues, tossing some blindingly fast licks around between fiddle, banjo, guitar and mandolin in the process. Down through the decades, bluegrass evolved as much as country music itself, and a new generation of pickers informed by rock, folk and jazz started gaining ground by the start of the '70s—the name of one of the movement's key bands, New Grass Revival, says it all. By the '80s the genie was really out of the bottle, with idiosyncratic talents taking bluegrass all over the sonic map, be it the pop-friendly croon of Allison Krauss or the funky, eclectic adventures of banjo virtuoso Béla Fleck. But for all of bluegrass' far-flung branches, the roots of the tree still reach down to those deep Kentucky traditions.