In this paper, we introduce accountable virtual machines (AVMs). Like ordinary virtual machines, AVMs can execute binary software images in a virtualized copy of a computer system; in addition, they can record non-repudiable information that allows auditors to subsequently check whether the software behaved as intended. AVMs provide strong accountability, which is important, for instance, in distributed systems where different hosts and organizations do not necessarily trust each other, or where software is hosted on third-party operated platforms. AVMs can provide accountability for unmodified binary images and do not require trusted hardware. To demonstrate that AVMs are practical, we have designed and implemented a prototype AVM monitor based on VMwareWorkstation, and used it to detect several existing cheats in Counterstrike, a popular online multi-player game.