Head-mounted displays and 360° videos have become increasingly more popular, delivering a more immersive viewing experience to end users. Streaming 360° videos over the best-effort Internet, however, faces tremendous challenges, because of the high resolution and the short response time requirements. This survey presents the current literature related to 360° video streaming. We start with 360° video streaming systems built for real experiments to investigate the practicality and efficiency of 360° video streaming. We then present the video and viewer datasets, which may be used to drive large-scale simulations and experiments. Different optimization tools in various stages of the 360° video streaming pipeline are discussed in detail. We also present various applications enabled by 360° video streaming. In the appendices, we review the off-the-shelf hardware available at the time of writing and the open research problems.