Straw is the dry stalk of a cereal plant, after the nutrient grain or seed has been removed. Straw makes up about half of the yield of a cereal crop such as wheat, oats, rye or barley.

In times gone by, it was regarded as a useful by-product of the harvest, but with the advent of the combine harvester, straw has become more of a burden, almost a nuisance to farmers. Chemically straw is composed mostly of cellulose and lignin, the same major components of wood. A wall built with straw bales can have a R- value somewhere between 40 to 50.