3D printing - or additive manufacturing - is a challenging dual-use technology: One and the same device can print toys or guns. Marco Fey explains the basics of the technology, describes its current applications, and provides an overview of its global diffusion. He shows the potential of this emerging technology and analyzes its possible risks with respect to the proliferation of small arms, majo weapon systems, and even weapons of mass destruction. He investigates how 3D printing might pose serious challenges to national and international security. The author discusses first proposals of risk mitigation through safeguards, export controls, cyber security, awareness raising, and industry self-regulation. Marco Fey is a research associate at PRIF focusing on emerging technologies and arms control.