Planetesimals represent turning points in planetary formation, when the materials required for building planets are first incorporated into bodies with radii from tens to hundreds of kilometers or larger, and are sometimes differentiated into metallic cores and silicate mantles. These early celestial bodies are the accretionary step between the dust of the planetary nebula and the cadre of rocky planets. Thus, planetesimals hold the keys to understanding how Earth was formed, when water was deposited on Earth, and why Earth and other rocky planets may differ in composition.