The Research+Practice Collaboratory provides a compelling case that illustrates the ways in which the collaboratory structure--which is distinct from partnerships, centers, consortia, and other common structures--can make Grand Challenges in education "investable." The collaboratory structure, we suggest, has features that allow foundations (in this case, the National Science Foundation) to invest resources effectively in challenges that inherently require combined effort involving a range of expertise, varying perspectives, and even multiple disciplines. A collaboratory structure supports "collaborating laboratories" that are more autonomous than those in centers and partnerships, are more connected to shared theory and practice than those in consortia, and more powerful jointly than independently funded laboratories. Collaboratories, well designed, thus optimize the benefits of autonomy while at the same time achieving many of the benefits of collaboration. [This report was written with assistance from Jenifer V. Helms and Laura Stokes.]