The delivery and display of ultra high resolution 360-degree videos on Head-Mounted Displays (HMDs) presents a number of technical challenges. 360-degree videos are high resolution spherical videos that contain an omnidirectional view of the scene, however only a portion of this scene is displayed at any time on the user's HMD. The delivery of such videos wastes network resources since most of the pixel data are never used. With high refresh rates, HMDs need to respond in less than 10 ms to head movements. The required ultra low-latency response prevents dynamic adjustments of video quality at the server based on client feedback. Instead, an entire 360-degree video scene needs to be delivered to the client to extract the appropriate fraction of this scene on the client's end. To reduce the required immense video bit-rate, while still providing an immersive experience to the user, a viewport-adaptive 360-degree video streaming system is proposed. In this system, the server prepares multiple video representations that differ not only by their bit-rate, but also by the qualities of different scene regions they support. The client chooses a representation for the next segment such that its bit-rate fits the available throughput and a full quality region matches its viewing direction. We investigate the impact of various spherical-to-plane projections and quality arrangements on the video quality displayed to the user, showing that the cube map layout offers the best quality for the given bit-rate budget. An evaluation with a dataset of users navigating 360-degree videos demonstrates that segments need to be short enough to enable frequent view switches.