The use of IP to transport voice is receiving significant attention in the telecommunications and data networking industries. Commonly known as Voice-over-IP (VoIP), this technology has the potential to allow user communication involving multiple media types, and between terminals that offer improved capabilities and user interfaces. Meanwhile there has been explosive growth in the demand for wireless connectivity, raising the question of how VoIP can be extended to situations where users and terminals can be mobile. In this paper we survey the technologies necessary to provide mobile VoIP, and identify relevant technical challenges.