Abstract: After an initial epoch of assembly, spiral galaxies like the Milky Way evolve primarily under the influence of slow, internal processes. This secular evolution rearranges the orbital angular momentum and energy of a galaxy’s disk, thus altering its kinematics, morphology, and chemical distribution. Central to our understanding of secular evolution is how transient spiral arms cause stars to migrate large radial distances from their birth radii. Radial migration is often associated with increased orbital eccentricity, thus kinematically heating the disk. However, a particularly important type of radial migration, called churning or cold torquing, can change the sizes of stellar orbits in the disk without significantly altering their eccentricities. The relative importance of cold or heating radial migration significantly impacts how disks evolve. In this talk, I will demystify the physics that governs various forms of radial migration and discuss their observational signatures. I will then present scaling relations for the efficiency of cold torquing. Finally, I will argue that in some limits cold torquing can, in fact, kinematically heat the disk. First steps have been taken, but there is an ongoing need to further develop this theoretical framework for the interpretation of data from high resolution simulations and large, high precision observational surveys of the Milky Way.