Additive manufacturing involves a new class of cyber-physical systems that manufacture 3D objects incrementally by depositing and fusing together thin layers of source material. In 2015, the global additive manufacturing industry had $5.165 billion in revenue, with 32.5% of all manufactured objects used as functional parts. Because of their reliance on computerization, additive manufacturing devices (or 3D printers) are susceptible to a broad range of attacks. The rapid adoption of additive manufacturing in aerospace, automotive and other industries makes it an attractive attack target and a critical asset to be protected.This chapter compares emerging additive manufacturing and traditional subtractive manufacturing from the security perspective. While the discussion compares the two manufacturing technologies, the emphasis is on additive manufacturing due to its expected dominance as the manufacturing technology of the future. The chapter outlines the additive and subtractive manufacturing workflows, proposes a framework for analyzing attacks on or using additive manufacturing systems and presents the major threat categories. In order to compare the two manufacturing paradigms from the security perspective, the differences between the two workflows are identified and the attack analysis framework is applied to demonstrate how the differences translate into threats. The analysis reveals that, while there is significant overlap with regard to security, fundamental differences in the two manufacturing paradigms require a separate investigation of additive manufacturing security.