Most books on missile proliferation focus on the spread of ballistic missiles or cruise missiles, not both. This book, however, explains why cruise missiles are beginning to spread widely, but does so by explaining their spread in the context of ballistic missile proliferation. It therefore treats both ballistic and cruise missile proliferation as related phenomenon. This work also focuses even-handedly on both non-proliferation and defence policy (including missile defences and counterforce doctrines) to fashion a set of integrated strategies for dealing with ballistic and cruise missile proliferation.Signs of missile contagion abound. In this study, the author argues that a series of rapid and surprising developments since 2005 suggest that the proliferation of missiles capable of delivering either weapons of mass destruction or highly accurate conventional payloads is approaching a critical threshold. The surprising fact is that land-attack cruise missiles, not ballistic missiles, constitute the primary problem.Flying under the radar, both literally and figuratively, land-attack cruise missiles add a dangerous new dimension to protecting worldwide security interests and preventing regional military instability. The author asserts that cruise missiles are not destined to supplant ballistic missiles; rather, they are likely to join them, because when both are employed together, they could severely test even the best missile defences. This work explains why an epidemic of cruise missile proliferation, long forecasted by analysts, has only recently begun to occur. After first assessing the state of ballistic missile proliferation, the author explores the role of three factors in shaping the spread of cruise missiles. These include specialised knowledge needed for missile development; narrative messages about reasons for acquiring cruise missiles; and norms of state behaviour about missile non-proliferation policy and defence doctrine.