Spam-over-IP telephony (SPIT) will likely have a significant impact on the usefulness of VoIP telephony solutions, but some solutions to the problem, such as filtering, could raise unanticipated legal issues.This paper contains both an overview and an assessment of the emerging legal issues in this domain and compares the legislation of two countries with very different legal systems: the US and Germany. Although call filtering addresses all kinds of attacks, we focus here on SPIT. Filtered messages that are part of a denial-of-service (DoS) attack or that attempt to exploit device vulnerabilities are much less problematic, and legal scholars agree that service providers have the right to defend against them.