Each of these technological advances has challenged copyright law. We focus on litigation when millions of users downloaded the programs from the Internet and proceeded to use them to facilitate widespread file swapping of copyright-protected songs. Such peer-to-peer (P2P) file-swapping has now become a cause celebre for owners of copyright-protected music, and to support legislation to suppress the new P2P technology. The problem of this demand for the imposition of vicarious or indirect liability is by no means unique to copyright infringement. It is just one instance of the old and general legal problem of how to decide whether one person is justly accountable for the wrongs that another person commits or the so-called accidents that harm another.