I will summarize recent progress on a longstanding problem in cosmogony, the formation of planetesimals. The first generation of solid bodies wider than an kilometer, planetesimals are not only the crucial building blocks for terrestrial planets. They are also of great interest for the Solar System's asteroid and Kuiper belts, for the debris disks they generate in extrasolar planetary systems and for the volatiles they can deliver to habitable zones. The great debate in planetesimal formation focuses on the relative role of collisional sticking and collective gravitational collapse. I will describe the Streaming Instability, a promising mechanism that spontaneously triggers the clumping of heavy solids as they settle into the disk midplane. The large planetesimals, > 100 km in scale, engendered by the Streaming Instability is causing profound reconsideration of Solar System planetesimals and planet formation in general. While direct observations of a forming planetesimal are inaccessible, multifaceted constraints --- which arise from microscopic study meteorites, collisional experiments in microgravity, the statistical ensemble of exoplanets, and the mapping of protoplanetary disks (currently being revolutionized by ALMA) --- make planetesimal formation a rich and timely problem.
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