I will describe our new approach to studying the kinematics of the Milky Way, Kinetic Tomography, and what it can tell us about the Perseus arm. Kinetic Tomography is the process of reconstructing the 4D structure of the Galactic ISM, by combining velocity information from CO and HI surveys with distance information from 3D dust maps. I will show how our new 3D velocity maps, along with a complementary approach using an IR diffuse interstellar band, can be applied to the study of the nearby Perseus arm. I will show how comparisons between our observed velocity structure and simulations of spiral arm formation allow us to rule out many theories of spiral structure in the outer Galaxy. I will also show how the velocity structure in the second quadrant creates a "Perseus Illusion" that makes kinematic maps of the arm strongly overestimate its strength, and contend that Galactic gas maps based on strong assumptions about the rotation curve are likely to be misleading.