Streamers are filamentary plasmas that are commonly found in transient electrical discharges. Research work on streamers is important for safe operation of high-voltage equipment and transmission lines in power systems. Streamers are also key components of electrical discharges in the earth’s atmosphere, such as lightning, jets and sprites. Despite hundreds of years of research, how lightning starts inside thunderclouds is poorly understood. The problem is that the measured thunderstorm electric field is well below the threshold value for electrical breakdown of air. About thirty years, high-altitude discharges called sprites and jets were discovered, and we now know that streamers are the building blocks of those discharges. This talk will begin with an overview of high-altitude discharges and then discuss recent streamer studies to understand the initiation of lightning and sprites.