This chapter discusses the ethics of a proof-of-concept demonstration of “parasitic computing.” A “parasite” computer attempts to solve a complex task by breaking it up into many small components and distributing the processing of these components to remote computers that perform this processing without the knowledge or consent of those owning the remote computing resources. This is achieved through the use of the TCP/IP Internet protocol and, in particular, the checksum function of this protocol. After a discussion of similar exploits, the ethical issues involved in this demonstration are analyzed. The authors argue that harm should be the standard for determining if parasitic computing is unethical.They conclude that a revised notion of the rights of ownership is needed when dealing with the shared nature of the Internet. Suggestions for future research are offered.