Weft or woof is the yarn which is shuttled back and forth across the warp to create a woven fabric. It can also be referred to as woof and in North America, it is sometimes referred to as the "fill" or the "filling yarn.".

The weft is a thread or yarn of spun fibre. The original fibre was wool, flax or cotton. Nowadays, many manmade fibres are used in weaving. Because the weft does not have to be stretched in the way that the warp is, it can generally be less strong.

The weft is threaded through the warp using a shuttle. Hand looms were the original weaver's tool, with the shuttle being threaded through alternately raised warps by hand. Inventions during the 18th century spurred the Industrial Revolution, and the hand-loom became the more robust spinning frame with the flying shuttle speeding up production of cloth, and then the water frame using water power to automate the weaving process. The power loom followed in the 19th century, when steam power was harnessed.