Apparently, these allegations are nothing new, but they may have been dragged into the spotlight recently by Howard Stern, who discussed the issue on his popular radio show.
In a nutshell, Zeppelin is thought by some to have stolen material from other musicians and used it without giving proper credit. Their first album appears to be the one that is attracting the most criticism.
Not surprisingly, this is an issue that can evoke strong emotions (and reactions) on the part of fans who defend the band and those who think Led Zeppelin should be condemned for ripping off other artists.
There’s a lot more information available regarding this issue that you can read all about by visiting Classic Rock Central where you will find quite a bit of information and various bits of evidence and whatnot.