Neck pillows can be filled with various materials depending on personal preference and comfort level desired. Some commonly used fillers include microbeads, memory foam pieces shredded foam, buckwheat hulls, polyester fiberfill or batting pellets, lavender buds, flaxseeds or other grains like rice or barley. Each filler has its own unique benefits and properties - microbeads offer good support whereas memory foam molds to your shape for customized comfort. Buckwheat hulls conform to the neck's contours and allow better airflow, while lavender buds provide a relaxing scent. Polyester fill or batting offers a plush surface, and grains like flaxseeds can retain heat when microwaved. Ultimately, the choice of filler depends on personal preference for firmness, support, and any additional features desired in a neck pillow.