Buckwheat hulls are the hard protective layers that house the seeds of buckwheat plants. It is a plant that despite its name isn't related to wheat, but to the rhubarb family. Buckwheat hulls (or sobakawa, in Japanese) have been used to fill pillows in Japan and other Asian countries for centuries, and have recently become more popular in the West as more and more people are realizing the health benefits of buckwheat pillows.
Buckwheat hulls hold their shape extremely well and do not compress into an unsupportive flattened mound as other pillow fillings such as cotton or feathers eventually do. Even foam pillows lose their shape or ability to bounce back after a few years. Buckwheat hull pillows are malleable and do not lose their ability to support the head and neck even over a long period of time. The hulls move and slide over each other to cradle the head and neck, but have very slight ridges on them that offer just enough friction to keep the shape your head and neck molds it to. As you move around at night, so do the hulls; they continue to reposition, and give you even support.