Breakthrough Starshot is an initiative by the Breakthrough Foundation to prove ultra-fast ultra-light nanospacecraft can be launched by laser radiation pressure to nearby stars, and will lay the foundations for a first launch to Alpha Centauri within the next generation. Designs for a 0.2c Alpha Centauri mission minimize beam director capital cost by accelerating a ~4 m, several gram diameter sailcraft for ~10 min. A number of hard engineering challenges remain to be solved before these missions can become a reality: Large coherent laser arrays are required. No consensus has been reached on the most suitable sail geometry for stable flight, “beam-riding”. The sail itself requires major advances in materials science and photonic design to produce materials with the required absorptance, emittance, reflectance, areal density and operating temperature. Along the way, the project will enable increasingly fast outer solar system and interstellar precursor missions. Breakthrough Starshot will pave the way for multi-lightyear pipelines of sailcraft that fly past each target star every few weeks. Beams such as Starshot will produce an extremely observable transient feature of Earth and therefore could be an observable of extraterrestrial advanced civilizations.