The authors discuss loose parts -- pipe cleaners, acorns, fabric, stones, and so forth -- as versatile materials not originally intended for children's play that they can manipulate, modify, and use in their play activities. The authors review the historical foundations of loose parts play, focusing on influential individuals and theories, and compare the prominent materials used in each approach to the current application of loose parts in early learning environments. They elaborate four key factors in play that influence children's participation, and they highlight some developmental benefits involving materials, space, relationships, and time. They conclude that loose parts play had a significant presence in children's lives throughout the nineteenth century and that a need exists for further research on the benefits to children of loose parts play.