Our understanding of planet formation advances quickly by studying extrasolar planets, disks around young stars, and relics of planet formation in our own solar system. The latter is found in the Kuiper Belt, a collection of frozen objects in the outskirts of our solar system. We discuss theories of planet formation and the role of collisional breaking, and show how some of them can be tested by Kuiper Belt observations. Finally, we present the recent detection of the first sub km object in the Kuiper Belt, and show that it indicates that collisional fragmentation is ongoing today in the belt, destroying objects smaller than about 40km.