Flying snakes (Chrysopelea) uniquely glide using aerial undulation. However, the functional implications of aerial undulation are unknown, as undulation may influence performance by increasing the surface area perpendicular to the flow, and enhance stability by redistributing aerodynamic moments. Here, we combine new kinematic measurements of gliding snakes with reduced-order modeling to determine the effect of aerial undulation on glide performance and stability. We recorded glides originating from a height of 8.2 m in a large indoor arena at Virginia Tech using a 23-camera motion-capture system (179 fps, Qualisys Oqus 500) and two high-speed cameras (500 fps, Photron APX-RS). Of 131 trials, we analyzed 45 trials from 13 individuals in detail. The increased temporal (4X) and spatial (6X) resolutions compared to previous studies enables us to reconstruct the snake’s time-varying undulation waveform. Additionally, by assuming an anatomically-relevant limit of vertebral twist, we estimate the local airfoil orientation throughout the body. From these data, we found that the snake employs a large-amplitude out-of-plane movement that is twice the frequency of the lateral wave, which results in changes in airfoil orientation along the body. To test the effects of different waveforms on stability, we used the kinematics results as input into a passive dynamics model combined with theory about glider dynamics. We find that undulation passively stabilizes roll and yaw moments, but overall, the snake is likely unstable in pitch. Additionally, we find large inertial moments about the yaw axis, which suggests that snakes can maneuver by employing a transient bias to the undulation waveform. Supported by NSF 1351322.